I have never hired the basement doctor, but I had them estimate a job for me once. Their price was twice what I paid J&J Waterproofing for i-beams. That's probably why so many realtors, myself included, recommend them j&j. Their number...(614) 329-8088.
I would agree that if it is structural, the cost will be way more than $500. If you are at all concern have a structural engineer take a look. Just know that once you have the engineer's report, it will be much harder or impossible to get financing if the seller doesn't fix the problem. Banks don't seem to like to lend on things like this these days.