Harmony Pro is under $500 at $300 but no open API or SDK as far as i know. I could be wrong because i have zero reason to care about API and SDK so it might be available.

I don't think you will find anything boxed for you that has lots of flexibility for "well below" $500. I never got into them but i know guys on here used to post about these $30 IR remotes they got from radio shack that they could upload custom programming to. I doubt anyone has an objective detailed review.