Gearxs.com has several wireless routers in the $20-30 range.
I think I paid $28 the other day shipped for one I sent my daughter.
I use file sharing a lot on my network. I have one machine that is basically a server and has a RAIDed backup set that also has music and TV files on it. I used to "sneaker net" files around on diskettes but the size of files made that impossible. I went to a SCSI drive in an AS/400 "shoe box" but that requires SCSI cards in all of your machines. Once I got my replay TV a router was necessary and the network grew from that. I usually have 8 to 10 machines connected at any given time. There are 4 in the living room.
BTW a network printer is probably the first thing you will want. You can get one of those little print servers that will hook to just about any printer.