In this case, that should be part of the hiring risk that the company takes on, else you're punishing the good developers, who presumably you want to join your company, for something they have very little control over.
Tell your barber that you're going to pay him 1/2 from now on because half the other barbers in the city are worthless...
You've got bigger problems, tho. A headhunnter fee is 30K.[1] Take 30K and divide it by 50, and you have over 600 hours to play with. Lets save ~85% and take 100 hours as your budget @ 2x the cost. That would cover how many work-sample tests? (take your pick) 20 peple x5hrs or 24x4hrs which should be a good funnel. What's wrong with this approach?
[1] Assume a 100k position., which is also $50/hr on 2000 hrs/year.
I think OP was asking from the seller's (interviewee's) perspective. $25 doesn't buy a lot of time from a software engineer. And from the hiring side it seems like you could easily justify a higher amount based on time saved on interviews.
Tell your barber that you're going to pay him 1/2 from now on because half the other barbers in the city are worthless...