this is where and the type of powerline adapter i purchased:http://www.newegg.com/app/ViewProduc...129-214&depa=5
if your cable modem is near your desktop, then you could set it up as the linksys rep described (you would need a router with more than one ethernet port though). otherwise, you could plug the router into your cable modem. then you would connect a powerline adapter into the router. you would then attach each computer you wanted to have an internet connection to its own powerline adapter. think of the powerline adapter as a replacement for ethernet cable. instead of running ethernet cable around the house, you replace it with powerline adapters. phoneline would work on the same principles also, but most people tend to have more electrical than phone outlets in the house. as ewolff mentioned, the throughput is rated at 14 Mbs, but real world is about 8-10 Mbps max.
at the time i purchased the gigafast adapters, these were the only ones under $50 i could find. now it looks like newegg carries a few more that are much smaller than the items i purchased for about the same price ($40). your cost estimate is about right. newegg carries many routers under $30, but i would choose one from linksys or netgear. i have found the configuration interface easier to use/access and had more features than other brands i have used.