We're still new to this, but we have tried a few interviews where we pay the candidate $80 an hour (~$160k salary annualized). On average, good candidates complete the projects in about 3.3 hours, so that's a ~$270 expense to us, which we've been fine with.

Candidates who did the unpaid version saw the clear value: they invest 3-5 hours once, and potentially cut out 20+ hours of first round interviews, or even all the studying like the OP did.

Nobody that we've tested in the beta complained about a few hours of free "work" - they were actually excited about it.