Ok... here's the deal with sadies. It's supposed to be switched roles, so naturally the girl asks and the girl buys the tickets. There are a few tricky parts here though.
a) Dinner - If you guys go to dinner (many times a typical dance deal) there's a problem here. Guys normally order more expensive than girls. Time and time again I've been to dances and seen steaks all around on the guys, and salads all around on the girls. The problem here is about double the price, depending on the restaurant. So, if you're comfortable with ordering something cheap, then go for letting her pay. I was told that proper etiquette is to order something under the price of the person that is paying. However, if you're hungry for something more expensive, tell her you got your meal, and stand up for it, and get what you want.
b) Pictures - I know at our school they normally have pictures of the couples taken at the dances. On a typical night, guys ask, guys buy the tix, guys buy the dinner, the girl buys the pictures. So, if everything's switched around for sadies, then go for buying the pictures. Otherwise it's quite a financial burden on her ya know?
C) Dancing?? Well, I'd say there's a 95% chance that everyone there will be freak dancing, so that's prolly what ya gotta get yourself into. Umm... it takes a little practice, and your legs will probably get tired, but if you don't know how, just basically stand right behind her, with your crotch against her ass, and hold on to her hips, but don't push her anywhere. Just let her do the moving, and try to follow along. This only works if she's a) willing to freak with you, b) you don't get in trouble *chuckles* and c) she knows what to do. Otherwise, you might be out on a limb there.
Hope that helps.