well, i don't think there is a cheap solution to what you want, but what you can do is get a linksys router that is in the WRT54g family and obtain the sveasoft custom firmware for it. I don't think it will let you split the bandwidth but it will let you prioritize your traffic over his

so as long as you dont hog the connection he should be OK. Also I think there is a program called NetLimiter (for Windows, not sure about Mac) that will let you limit the bandwidth of traffic going in/out of a PC. So I guess you could install it on your PC set the limit to be half the bandwidth available to you, and then give yourself priority using the Linksys router. It's kind of a clumsy method but it's all I can think of if you want to build it on the cheap.