The forum for Spine isn't a great place to ask about alternatives. 😃
Obvious bias aside, you may be able to find cheaper solutions that have worse support and fewer features, but there aren't any with the same or better support nor with more features. Here you get a personal response from the owner of the company on Saturday, within hours of your post. 🙂
Spine's benefits go beyond that though, for example by having good workflows specific to skeletal animation for games. Other solutions, like Unity's 2D animation, are tacked on to the side of a larger solution rather than being focused specifically on solving 2D skeletal animation well.
Spine also has good architecture for its runtimes, providing the features and flexibility needed in games and doing it with efficient, easy to understand code. Check out the runtimes guide and runtimes source code.
Hands down, the best way to evaluate the available solutions is to create a character in each software, export it, show it at runtime, and write code for some interaction (like clicking to play an animation). Do that same process with each solution and it will likely be quite obvious which one to go with. While the Spine trial can't export, you can use the example project exports to try out animations at runtime.
