How much does it cost to register?
Well as long as you pay an internet bill, technically nothing.
I pay (4 my 56K) $20 a month and I could set up a server for my nextdoor neighbor and leave it up all night and next months bill will still be $20.
It also depends on your isp. Charge by min, certain amt of hours. Cable is no worry. ?? DSL I don't know but I imagine its the same as cable.
Setting up a server is easy. Another way to check your ip is after its aleady running, open up another (if you ran a dedicated, if not don't open another) UT an search for net games, find you name (granted you named your server) and viola!