I have a few betting and horse racing domains and for me I don't think these ones have a great deal of value undeveloped.
The main keywords themselves aren't bad i.e. horse betting and horse bet. If you can find an end user you might get more money, but seeing as there are equally as good domains available with similar or the same keywords it would be quite hard.
Development may add quite a bit of value if done well and you are good at SEO as the keywords have some competition.