I am not sure a "not cheap" $500 would be near enough!
A pre-purchase with a written report with some travel would be a very long days work.
A pre-purchase is pretty much an annual inspection + a very in depth look at the log books + AD/Bulletin search and compliance. This is often followed up with quotations or approximations for compliance or rectifications and numerous follow up questions (email/phone).
The reality is it is probably around two days labour (16 hrs) + transport and parts (oil filter) all for $31.25 per hour!
$31.25 per hour will not get what is required - I do not even think it covers the award wage for a qualified person. I would expect an individual would want at least $50 per hour + costs and a company more.