As a general rule, if you play up the 'thoughtful' angle, us girls ingore the cost of the item (if yours doesn't, don't lend the goldidgger your credit card.)
If you're lucky enough to be living someplace warm, see if there's a field/roadside around with flowers in bloom. Go pick the flowers yourself- it'll be free, and she'll love that you went to the effort. (If the area's public/scenic enough, go have a picnic.)
If you opt to buy flowers, consider getting her birth flower instead of the usual roses- look
here to find out what to get (they also list birth stones, if you want to try for a cheap jewelry thing). It'll probably cost less than roses (unless she was born in June, than you're stuck with 'em), and you get to be 'thoughtful' for figuring out the birth flower thing. You can save a fair amount getting silk flowers instead of fresh if she's OK with that- I know it sounds cheap, but there's some really nice silks out there now- try a craft store.
Actually, that's a good idea in general- there's lots of cute lil' things at the craft store I work at... you could buy a dollar bead kit and make a bracelet, mold your own chocolates, get some nice card-making stuff (none of this is very hard to do, talk to the clerk for tips.)
For the girls- just try to cater to the guy's interests, even if it seems 'off' for the holiday- I got my BF an Xbox Live points card and a couple small candies. I really,
really thought about getting a Hilter/Nazi documentary DVD, but couldn't decide what specifically to get. You can also try making cookies or something- I've got a recipe for homemade buttercream frosting if anyone wants it. I made that one year with some sugar cookies (made with a just-add-water mix from the grocery store, cost a buck), it went over well.