This isn’t SUPER-cheap, but when I was a kid, we spent $20 to buy a used homemade ping pong table that we had for 15 years or so. As far as strictly material posessions, that has to be in first place in the dollars-spent-to-happiness-received-ratio category.
More recently, and this is more a question of value than cheapness as you’ll see, I have gotten incredible value from purchasing a TiVO with a lifetime subscription. It’s just so worth it.
Paid zero dollars and zero cents for a Louis Armstrong concert in my younger days. Sneaked in.
The $3 I spent for used-bookstore copies of my two favorite books in the world. I hadn’t been able to find them in a big chain bookstore and was about to resign myself to ordering them online. Tried the one used bookstore within a reasonable radius and there they were. The only thing I don’t understand is why anyone would want to get rid of those books, unless they bought themselves nicer copies and got rid of the old ones for that reason.
When my son was just about 13 months old, I spent less than $2 on a strawberry sundae for him from DQ (maybe $6 total, if you count my blizzard). Watching him try to eat that by himself was amazing and hilarious and cute and fun.
Oh màn! You’re married already. 
That was so cool.
I got my dog (choc lab) for a six pack of beer.
I guess I like my dog alright.
(However, over the years he’s cost me a LOT more than a six pack.)
New floormats for my car. They fill the whole car with new-car smell for weeks.
Probably some sort of deadly fume, but I like it.
Wow! I have officially been flirted with!
Thanks, gum.
BTW, I incorrectly stated the name of the organization to which my wife donated over a foot of her hair: Locks Of Love. If you’ve never heard of it, it’s set up to make wigs for children with cancer, who have had chemotherapy and lost their hair.
At a children’s consignment shop, since I had no cash, my friend fished out $0.95 for a goofy little Incredible Hulk mini soccer ball for my daughter. She *loves * that thing, and will often drag me into the playroom so we can kick it, throw it, and play keep-away.
Wow, I didn’t even think about pets. I’ve had a bunch of dogs over the years, and I’ve never paid a dime for any of them. I’ve spent plenty on vet bills and such, of course, but I’ve never bought a dog. And dogs are the greatest thing ever.
I have a million-dollar dog that I paid $11.00 for. She’s a Basset Hound named Shilo, and we got her at the pound. She had been turned in by her owner because another dog was savaging her. She only weighed 35 lbs. when we brought her home, and had bite marks around her neck and through her ears. She is the sweetest, most loving dog I have ever had. I can’t believe someone would give her up and keep the mean dog, but I’m sure glad they did. She is now officially The Most Spoiled Basset Hound In Existence. And she weighs 52 lbs. as of her last vet check. 
I really don’t want to sound like I’m demistifying the event, but the small change (£1.50) I spent on a bus trip to meet my girlfriend at her place and lose my virginity with her must rate quite highly
After that its the assorted video games and books, bought second hand, that went on to provide hours of fun 
$0.35 for a kazoo, at the Alameda Flea Market. Rode back across the Bay Bridge with Carl, music all the way. Kazooing was a new world of delight.
There have been other good cheap joys too.
I spent around $3.50 to take bart to meet an online friend who was to be my current SO. Almost four and a half years and still going strong!
I would like to add to my list.
Ever since I started taking nutrition seriously I have noticed a good increase in my physical and mental health. I don’t get normall illnesses anymore (if everyone around me has a bad cough for a week I get a sore throat for a few days) I am not as depressed anymore and the depression doesn’t last as long, and my sleep is alot better. The cost of these supplements runs me about 40 cents a day. Probably the best money I’ve ever spent.
Fish and Other Fish cost 25 cents each. Don’t tell my other, now dead, fish, but these two are the best goldfish I’ve ever met.
A few years ago I went to a Scholastic sale for educators and found some really neat tops for a dollar each. They were soooo cool, you twisted them, then pushed a button to release them and they lit up and played music. I bought 13 of them. A few days later we had a Christmas party at the special needs kindergarten, and watching the twelve kids (see, one top was for me) play with them was great - most of them liked my present the best, lol. Even the student I was an aide for, who had autism, was facinated by his. $13 for a great afternoon was really a steal.
Two things,
Once when I was travelling through Indonesia, on an all third class train (mistake) on a sweltering tropical evening. I purchased, for the princely sum of, 7 cents, canadian, a small hand fan. It was a 15+ hour journey and the fan was some of the wisest money I spent for pleasure gained. Damn fine value.
And, as for baubles,
At a boot sale in Singapore, I bought a curious, round, brass, velvet lined, well I don’t really know what it was really made for. Pill box maybe. About the size of a golf ball with an unusual clasp and detailed edging. It was very lovely indeed. I believe I paid less than $3 Singapore for it. Everywhere I took it, in my pocket or in my bag, if I took it out, people were drawn to it, just as I had been. It was a pleasure to hold in your hand, a lovely weight, polished and warm to the touch. I have it still but the clasp has broken and I haven’t yet found anyone to fix it for me though I have tried two jewellers. The expense to fix it will far exceed the price I paid for it, but it will be so worth it.
I’m shocked nobody has said “$7.48 for the SDMB subscription.”
I’m going to have to challenge you on that one. Our Bassett, Katie, was found wandering in a park where she had been dumped after apparently being abused for some time. The folks who found her couldn’t keep her, so they passed her on to some friends. They didn’t have enough time for her and were going to take her to the pound when we got involved.
She cost nothing, and I regularly lift her on to the bed so that she can sleep at our feet (although she seems to prefer sleeping on he pillow). She gets my vote for MSBHiE.
Well, Shilo has her day bed in the living room, her night bed in our room with an old cotton blankie, a dogport (no sides, but raised platform with a shed roof) with pad in the backyard, and another dog bed in the garage for when we have inclement weather and she can’t go outside while we are at work. 
But they are the best. dogs. ever. I’m glad your Katie found a good home!