I'm pretty sure I would get into a significant amount of trouble with my company's people team if I accepted $50 from a candidate. I honestly would consider the presence of this offer on your website as a red flag.
Yeah, if I really wanted to know if someone had clicked through to my website, maybe I'd put something like "Ask me about the time $THING" But, really, an interview doesn't seem like the time to play cutesy games.
You don't have to accept it, you just have to mention it.
OTOH, I'm pretty sure most companies have rules against hiring candidates who offer money to the interviewer (even if the interviewer declines, the candidate is rejected anyway).
