I do think it is awfully presumptuous to think everyone in your circle of friends can shell out $200+ for a birthday party! Maybe for family or a landmark birthday (like turning 80 or something) but just to ride downtown into a football crowd?! I'm with you thinking that is an obnoxiously overpriced thing to do in February in Michigan!
Now, personal opinions aside, I don't understand why you can't just be up front with this friend - not about the money necessarily - but about the physical problems that you are dealing with. Michelle, rheumatoid authritis is not a "light" diagnosis! Not to mention the fibro and everything else you are dealing with! Certainly there could be some measure of understanding between you without costing the friendship! Can't you meet them somewhere downtown? Or later at the b-day girl's house? I'd be honest, not concentrate so much on the money (although I would die before I'd shell out that much on a birthday party) but just be honest about your health. That, to me, is far more important. You have to be true to yourself and take care of your recovering body right now.
Just my two cents. Sorry you are having to deal with this!