Howdy,
Well, you have two options.
If you opt to buy it now -- and you later decide you don't like it, you can request a refund within 45 days of purchasing it. Joe and Jamie are pretty good about all that -- they want folks to be happy with it, so if for whatever reason it doesn't meet your needs, they'd certainly refund your money (though, be sure to inquire first, as maybe there's something they can help with!).
The other option is that there is a GPL/free version (Over at
http://webmin.com/vinstall.html). It doesn't have all the features of the Pro, and you'd later need to go through a (fairly painless) upgrade process to switch to the Pro version. But it's certainly an option if you're reluctant to hit the "Buy" button ;-)
Have a good one!
-Eric