Here’s a voice for restrained elegance and a minimum of dollar store paraphernalia. I do not really understand why people who are otherwise not into stuff get all into doodads and decor items for weddings. I’ll also mention that while spray-painting and creative crafting and such will give you custom doodads on the cheap, you will have plenty to do without those tasks – unless you really really dig doing that sort of thing for stress-release. That said.
If you or anyone you know is a big candle user, then candles could be the way to go, as said above, because there will be leftovers. You might consider using something to hold these candles that will also not be trash. Maybe you registered for nice china, but you could get a dozen plain white Corelle bread plates to set pillar candles on, and those could be your everyday don’t-care-what-happens-to-'em dishes. Or several sets of coasters. Or if you live anywhere where the power goes out a lot, get a bunch of candle lanterns like this or this or this or this.
To pretty up your tablecloth, you could get some yards of pretty fabric or whatnot. Gorgeous shot dupioni silk goes for around $20/yd online. A silk sari could work, too, for even less (saris are about six yards long and lovely silk ones can be had for well under $100). If you know any sewers, they’d be thrilled to get the yardage afterwards, even if you do spill a bit of wedding dinner on it in a few places (you can wash silk if you’re careful). Or super-wide ribbon, like 6" wide, available from craft stores and of course online in many kinds of pretty.
I like DiosaBellissima’s idea of paper lanterns over the table; they usually have a variety of those at places like Cost Plus Imports, and they can be found very cheaply available online.
If your fiance feels strongly about having fresh flowers, try talking her(?) into one statement-y vase of elegantly spare big ones maybe with a handful of smaller ones to scatter about, in simple bud vases or float bowls or something, rather than piles and piles of them.
Which kind of flowers might make a price difference, too; tulips like those in my first linked photo should be a lot cheaper than roses or anything exotic or short-lived. Also whether they will be in season matters.
Finally, find out, if you can, whether this was an idea planted by a Wedding Professional, such as a Planner or Florist. It sounds like it could be. As you surely know by now, Wedding Professionals, like car salesmen and funeral directors, have a unique customer base: customers planning a big cash outlay for a rare occasion with status on display. They will come up with all sorts of things you didn’t know you needed, and the Wedding version of anything has a massive Wedding markup.