I also have seen that a number of Pros have been contacted when I responded to a promoted job. If a customer reaches out to me, I am under the impression that they solely want to communicate with me.
I agree with what you said about decreasing the lead cost with the number of Pros selected. How about after 3-5 Pros contacted that the cost decreases 50%? Some platforms offer that, and I think it would work well for Thumbtack, this way if I'm Pro number 6 that a customer has reached out to, I'm not arguing over an average cost in my job category of say $30. Then I might be more inclined to reply, knowing that I have an outside chance of getting the job, but at least I don't mind spending half off the lead cost.
Don't know if that's what you are suggesting, but it's something to ponder. Maybe Thumbtack will take that to research for discussion?