How can i approach this situation?

ok first of all im anti social but that DOESN'T mean i dont have FRiENDS, i do i have alot i just dont socialize with them tht much but i talk to them on fb (atleast the ones who interest or entertain me). It was my birhday 4 days ago and one of my firends invited me to a "birthday dinner" to celebrate yesterday here is the problem, I'm a humble guy and i have never liked people spending money on me unless it was nessecary like clothes, hair gel, shampoo, shoes, ect. I'm an open minded individual so i dont mind if she pays for it like some would. i normally don't go out of my house to things like this since im a matter of facto person only doing things if its neccesary but if i say no it could hurt her feelings since i have known her for about 8 years. she could also take it that i dont want to hang with her as a happy birthday gift or somthing. what could this birthday dinner thing cover? since im a humble guy as i mentioned before im thinking of just going to a carls jr and calling it even so she will only waste like 5 bucks on me ( im too humble and considerate for my own good guess thats a good thing) but she could not like it any ideas?