The trial for Fractal Terrains is definitely worth playing with. Campaign Cartographer does not have a demo, but they do offer a 14-day money back guarantee if it doesn't float your boat. The Profantasy folks are very helpful and responsive, so I'd say you can trust that guarantee. It does have quite a steep learning curve, though, so if you don't have quite a few hours to spend during that two weeks you might not get anywhere with it. Fractal Mapper by NBOS has a demo version without an expiration date, although it doesn't allow you to save your maps, and it doesn't have the full symbol set included. I am given to understand that it's a bit more user friendly than CC3, but I can't speak to that personally since I've never used it. On the other hand, Campaign Cartographer is a little better supported, with multiple symbol packs and expansions, and a subscription service called the Cartographer's Annual that offers additional styles and mapping guides.

Personally, though, I think that if you have just a little bit of money to spend, you should spend it on a Wacom tablet for use in Gimp. You can sometimes find them for cheap on Craigslist, or you can save for a bit and buy a new one. The Bamboo Fun is the model I recommend for a novice. You can usually find them for around $150 if you shop around and are patient. You could also go with the Bamboo Pen or Pen & Touch; just stay away from the touch only models. Also, I can't in good conscience recommend any brand other than Wacom at the moment.