go buy the cheapest server you can (http://www.123systems.net/ they have been pretty reliable for me, most likely you only need 1gb or 2b max) that has enough performance for what you need.
IF your game suddenly happens to become huge and your cheap server cant handle it, you can just change to a new company where your getting a better server.
IF your game turns out that you don’t get the thousands of players you intended, you lost $50 (most likely $30 max) max for a years worth of server hosting to what you can recover from and try a new game and use the same server to host your game.
This $20-$50 you spent for 1 years worth of hosting saved you in electricity bill, which probably would of cost more if you used a home server based on the fact you had a spare computer. if you had to build a new computer to host the server, well you just saved a few hundred initial cost.