How are lead prices calculated?

I am just wondering how Thumbtack decides how much to charge for each lead. Lately the prices seem slightly out of control. For instance, I notice that the maximum lead price for Test Prep Services has jumped to $124, and if the past few months serve as any indication, this means most lead prices will now be close to this level.

A Test Prep Services job is only worth about $1,000 if you get it, and a lead only has about a 1-in-5 chance of turning into a job. Thus, a Test Prep Services lead is worth about $200 on average. If it costs $120, that means Thumbtack is taking about a 60% cut.

Is this intentional? It just seems really high, and I can't help but wonder if maybe this is some kind of accident. I've mostly had a good experience with Thumbtack, but these last few months have been discouraging.

I would feel better if we at least had some specific information about Thumbtack's pricing algorithm. Is such information available anywhere? Help and advice are appreciated, as always.