I agree with everyone that you ought to quickly and politely (since they do have your stuff) get your things out of there and end your financial obligation to your friends.
I don’t think you should send that note. While it may be true that you are ‘right’ about everything, as soon as you start bringing up justifications for your point of view and listing things you have done for them financially, you open up a dialogue that you cannot win. They will not read things like, “I saved you $1000 each,” and “I paid the verizon bill in full two times,” and think, "oh, he’s right, we’re being stupid. What they will do is see that as a call to war. They fired the first shot with a completely unreasonable request for more money. When you respond with financial details, they will continue to argue.
The cheapest, quickest, and most low-stress way to deal with this is to arrange for your stuff to be picked up and moved to a storage area (with as little notice as possible to the roommates), then get yourself off the lease ASAP. If these are truly your friends and you want to keep it that way, maybe pay one more full month of the $200 ‘rent’ after your stuff is cleared out, so they can’t complain about you leaving them ‘in the lurch’.
Then go on your merry way.
But, I would send a letter something like this instead:
==================================
Hey! As so far as the $211 is concerned, no problem. It sucks that that’s how the utilities are calculated. The check is in the mail.
I don’t think, though, that I can afford/justify the $300 dollars. I understand where you get the $300 a month from. A 10% increase (+20), plus the 80 for the sewage. Viola, 300 bucks… but the fact remains that I would be paying nearly 3 times the going rate for storage in the DC area.
So, I’ve arranged for [someone] to come get my stuff on [date]. After that I’ll still pay the rent on the closet for another [month/two months] so you have some time to prepare for that loss of income. And, I’ll get my name off the lease as soon as my stuff is out so you don’t have to pay the increased utilities.