What do you know about portable chassis dynos?

What do you guys know about portable chassis dynos? Are they as accurate as a stationary? Or are they off a little? I saw one at a show about 6 years ago and the guy was only charging $40 for one run. How much would you pay to get one run and/or what is the going rate now? I know a few places that have stationary dynos and they charge around $75 for one run.

ANY input is appreciated!! THANKS.

Local place charges $50 for a set of 3-5 dynoday pulls.
That is a set day though. Normally a little more expensive.

I don't see a portable being any less accurate though. They should still be calibrated.