There's no trial version of Remote Desktop Services, it's just a role that you add to the Windows Server you'll be allowing remote users to log on to. But there is trial versions of Windows so you can get it all set up and try it out before you fork over your money.
When you add the role, it has a 120 day expiry. After 120 days you have to plug in client access licenses to continue using the remote login feature. There are some sources out there that can tell you which bits to flip to get another 120 days.