For less than ~$100 in parts you can build your own. For example see the last page of
https://www.digikey.com/htmldatasheets/production/80467/0/0/1/ltc2485-datasheet.html.
The ltc2485 is a 24 bit ADC with an update rate just about right for a DPM, and it cost $7.00! I've used a ton of an earlier version and I can assure you they out preform the NLS voltmenter I had in my lab as a grad student.