Indicate then Mitigate
Underfloor slab moisture testing for flooring can be a daunting prospect, especially since the introduction of the in-situ RH% at depth method. No more simple moisture meters- turn it on, pop it on the surface- bang, you're done. And those larger jobs with major square meter-age makes it expensive as well as daunting.
An option taken by many organisations nowadays is what we euphemistically call "Indicative testing". This may not fully comply with the applicable standards (AS 1884-2012 at least), however it's a step in the right direction.
Far be it for us at FTA to encourage sub-standard practices, but hear us out: The tests can be conducted in all respects to a fully accurate and compliant manner EXCEPT for the number of tests required. This is where the cost builds up.
When you have slabs poured recently - and by "recently" we mean even 12-18 months old - you may only need an indication of the moisture condition of the slab in order to make the correct decision. Put simply, if the slab reads high for all required purposes, then you are closer to a decision to mitigate, and advise / quote the client therein, without needing to spend more on the full number of sensors required by the applicable standards.
Only if the readings come in low should you therefore need to consider extending the test locations to the numbers fully required and committing the costs accordingly.
Even then, remember the issue of standards vs manufacturer warranty requirements. Sometimes they aren't the same. Who's paying the bills? Who has the liability? It might well be that your flooring product manufacturer is satisfied with the tests conducted and is willing to warranty their product. If you have that kind of relationship with your suppliers, then take advantage of it and ask!
At the end of the day, it all leads to a successful, trouble-free flooring installation.