You are right, $100x3 ($200x3 including the offer from the pfsense team) is not serious money to audit that code. However, given how quickly some people are assuming the supposed backdoors to be real, I imagine some hundred bucks will be a good incentive for them to reveal their own sources and audits, based on which they arrived at their informed conclusions. After all, so far they're doing it for free.
If you look through the thread, there are a bunch more offers. It ends up being $1400*3 last I saw. That's a nontrivial sum.