Disc golf courses cost money. They require land, baskets, tee pads, signs etc. The question is who is actually paying for it? As I see it, there are four potential sources. I think most courses use some combination of the four.
1. Taxpayers - These are public courses. They are built with public funds on public land under the control of local government (usually a parks department). Since the locals are funding the course with their tax dollars, these courses can be made available for little to no fee.
2. Players - By charging a fee, you put the cost on the players who actually use the course.
3. Sponsors - By offering advertising around the course (such as on tee signs), course owners can get course funds through sponsorship. However, because a disc golf course is not particularly fertile advertising ground (small audience), it's hard to fully fund a course with advertising.
4. Owners - In a sense, this is the same as option 1 (where taxpayers are the owners), where someone with the land and financial resources decides to build a course and allow anyone to play for free. Outside of the public, taxpayer funded model, this rarely exists (if at all).
The money has to come from somewhere, and with more money comes better upkeep and amenities.