Indeed extremely low. First of all, you would need a good prior art search for a patentability opinion, which not only involves searching for earlier US patents and patent applications, but also searching for all documents, including scientific literature, foreign patents and may be even the internet.
A good prior art search would be priced from $1000-$2000 if you outsource, depending a bit on the subject.
Then you need time to spend on studying the uncovered documents and compare them with the invention to see if they anticipate. Lastly, you have to write things down in a language that is understandable to your client.
I cannot believe that all of this can be done professionally for only $500.