most service providers will give you a router if you don't have one. If you don't have one from them, you can get a cheap one pretty much anywhere that sells electronics. I assume you're desktop has a wired connection, and you should probably be leaving it that way, since wireless is slower and you're desktop may or may not have an installed wireless card.
Lets see.....Well depending on your cable company is one thing because you get tv and internet, but for router and adapter lets see....my family and I have a linksys which is a good wireless wifi router we use. It could be 40 to 50 if I remember right. Yet you gotta compare other ones and ask around to see if it'll be compatible and which one is reliable to give you good wifi service.