The most affordable and accurate UVB meter (for the price) in the market is, in my opinion, the Solarmeter 6.2 by
Reptile UV. I own one personally and have used it for zoological purposes (they are a staple in zoos and aquariums).
The price is $179.00, which is a great price for such investment.
Bob MacCargar is a fantastic individual and will answer all your questions promptly.
Good luck,
Fabián