ServerHex offers MTA:SA servers for 0.15 GBP per slot (£0.15). The minimum slots you can buy is 15 and the servers are hosted in the USA. With your server you get a MySQL database, FTP and a control panel with many features (includes auto-restart, server zipping, process viewing and sub-users, so your admins can restart your server or view the logs without being able to see other parts of the control panel or use your account!).
There is also a referral program which allows you to earn money towards your server. You get 30% worth of whoever you refer's first order into your account.