Okay, if you lived apart, I'd say surprise her with a nice dinner at your place. Even if you put take out on nice plates, it's cheaper than a big night out.
My advice is to:
1. Send her some flowers at work. Doesn't have to be roses. In fact, most girls I know prefer something a little different. The websites for flowers are pretty good. You can get something unique. Lilies are always pretty and they smell really good.
2. Take her out to a nice dinner. It doesn't have to be the most expensive place in town, just somewhere nicer than Pizza Hut. Also, I don't know where you live or what the climate is like, but if you're in a warm climate, I nice picnic in a local park would probably make her heart melt. Then you really only have to buy (or make) a few sanwiches and bring a bottle of wine (in a thermos if it's a state park).
3. Hope that her gift to you is slutty lingerie.
The thing is...and I know this sounds shallow...it's not just about you two. If her office is anything like mine, everyone is going to ask what you did for Valentine's Day now that you're engaged. Every girl wants to be able to answer with something so sweet and romantic that the other ladies in the office (plus her friends and family) can see just why she's so lucky to be spending the rest of her life with you. They're all living vicariously through her right now. Unfortunately for you, that means you have to impress all of them. As rational and responsible as it is, no one wants to say, "Well, we spent so much on the rings that we just had a quiet evening at home." It's just not romantic to think of things like money on Valentine's Day...no matter how smart it is.
Honestly, I don't know your fiance, but most women value the thought over the money. If you can figure out something so romantic and personal that no one even stops to say: "Hey, sounds like he only spent $10 on your Valentine's Day", you are golden.
Go over-the-top mushy. If there's any way you can get her out of the house for a couple hours on Valentine's day, how about setting up a surprise picnic in your living room? That way you don't have to break the bank, and if she REALLY likes it, you won't get arrested if she wants to show you how much right there on the picnic blanket. *wink* Get some candles, some take out that you know she loves and make a CD of all "your songs" for mood music. Sure, the guys around here will call you a homo....but you'll be getting LAID, so who cares, right?
The bottom line is: You know your woman. I'm sure you have it in you to make a really special day for her without breaking the bank. You don't have to remember her favorite flower or anything major. Start small. I know guys have a hard time retaining all of the cryptic little hints we throw at you. Bless you hearts for trying. A simple gesture that shows you pay attention when she talks goes a long way. Pick up her favorite candy bar for desert. It really is that easy...at least with girls like me. I don't know you're girl, so I can't say for sure, but I think my advice is pretty general. Like I said: You know your woman. If she's a sucker for sappy movies, get sappy. If you guys are always laughing, find a little gag gift to get her as well.
But definitely send flowers to work. We eat that shit up.
Does that help? I'm still not really awake yet.