It doesnt really look as a 500USD, keris. I'm sorry. Its rather a common type, might be a bit older, but that doesnt make it special. As mentioned before, you can find some prices of similar pieces in the closed items section of eBay.
Just accept the fact that you have helped the "guard" to support his family and you did a good thing by stimulating local economy.
I agree with Rick, dont buy from just anyone offering you a keris. Especially not in a tourist area (of course).
Disagree about the online auctions, I think you have some very good offers on eBay from respectable sellers. But be carefull who you buy from.
