Got a BOM - LHR leg in economy coming up on AIr India and they just sent an email offering a biz class upgrade. The way it works is that I have to put in an offer and if it gets accepted I get the upgrade. If it doesn't, I keep my original seat.

I'm flying on a United mileage ticket and I'm not sure how much would have a decent chance of securing the upgrade. Any data points I can benchmark against? I was thinking somewhere between $250-500 might be good for a 10 hour flight.