Man up; don't try to pull a fast one over her with some game that she'll probably think is stupid. Take her to her favorite restaurant and get her something you know she'll like, or something that shows you pay attention to her (like something she hints at or talks about). It doesn't have to be expensive, as long as it shows you listen and care. If you really want you can like name a star after her or something if she's into that sorta thing, I think it's like $50 or something. Kinda lame, but if she'd be into that (it is romantic, I guess), it could work. But also get her something you know she likes (it'd be better if it was something that she didn't obviously like, like I said if it's more obscure it shows you listen to her). Anyway, if she still asks about that huge thing then admit you were a dumbass and wasted your $ over the months but that you still wanted to do something nice for her anyway, something that you know she wanted to do. I think that's your best shot. By the way, good luck getting into her favorite restaurant on Valentine's Day, unless you've made reservations or she likes crappy restaurants idk who you're gonna have to shack up with to do this. Actually, it might be good if you just cook her something, throw together a little dinner for the two of you. Good luck with all that.
EDIT: If there's one thing I should make clear it's that you shouldn't try to pass off some game as the "huge surprise", she'll know you're grasping at straws and probably be pissed at you. The most important thing to do is to know that you listen to, care about, and appreciate her. Unless she's a b1tch, the amount you spend on her shouldn't matter as much as that.