Here is an example of a product aimed at the field service tech:
Sencore SLM1476CM
Here is the least expensive option I have come across which claims to offer signal level measurement calibrated in dBmV:
HDHomeRun Tech
I have not had hands on either of these products, so I'm not willing to recommend for or against.
I do own the 'non tech' version of the HDHomeRun. It offers relative Signal Strength, Signal Quality and Symbol Quality reporting. I'll add some screen shots of that feature in my existing thread
Re. the HDHR
If I were charging customers for antenna installation and tuning, I would feel obligated to invest in equipment that was calibrated and verifiable against industry standards. Both to be sure I was delivering quality service and to be able to defend against the inevitable claim. However, for my own use and free assistance to others, I've been able to glean more than enough information from the signal metering built in to most STB's, DTVs and my HDHRs. (TV Fool is also a huge resource
)