Dude, if you can't afford her Big Mac and diet Coke then perhaps you shouldn't be dating.
Haha... good one but ya know; those Big Mac's ain't as cheap as they used to be. Fast food is fast but it isn't necessarily cheap. Not Outback or Benihanna's expensive but not cheap either.
I went to one of those Benihanna type places in Vegas one time last year. I only got hot tea with it and the bill for just me was $90 with tip. I like to treat myself once in a while but not quite that well.
I figure the actual food cost for the restaurant was under $7; the rest was profit or to pay the help.