I Just got asked to be a date at fraternity formal

[greekintraining]

So, one of my great guy friends who just finished pledging has asked me to his fraternities formal. 1) Would it be wrong to assume that he should pay my way? 2)Are there any online stores that would sell dresses (nothing to fancy) for a reasonable price (lets say $50-$65)? 3) He's a wee bit shorter then me, so is it ok to wear flats or maybe a shorter heel? (In case its not clear I am a girl and he is a guy)