For a single peptide test, I've paid from $5 to $15. Obviously depends on the number of participants in the test. There are tests with multiple peptides from a single vendor or multiple vendors which tend to cost more.
To test a product, simple post a test request to the request channel and when enough people show interest a dedicated test is created for group discussion and recruitment of additional participants.
The vials donated for testing are picked at random among the participants who have the product and volunteer. I believe 4 vials is the max needed for testing. For those picked to donate a vial, I believe the cost of the test is reduced or the Donor is refunded an amount equal to that vial cost. A prepaid postage is provided to the donors to send to one person who send all the vials to janoshik lab. The admin for the ptds coordinates the activities and receives the test results, which are shared only with the participants, unless the test results are unfavorable, or below a certain threshold, then it is publicly shared for harm reduction purposes.
That's my general understanding of the process. I may have a few details incorrect about donation compensation, haven't yet been picked to donate. I've also only been involved in the PTDS for about 3 months and have participated in maybe 12 tests to date.
Given the current state of the seller market, people are very open to test new vendors and find that next Xce type vendor. For established vendors, testing continues to make sure the quality continues to be acceptable and safe.