Assuming he goes (which he absolutely should, I'd be shocked and amazed if SG kept him). The fee will probably be heavily performance-based and the upfront fee will be fairly low I'd imagine as low as 5M and all the way up to 10M which is probably unlikely. I think the add-ons will be semi-realistic and with his record I'd go with a sliding scale e.g. 5 EPL goals 500k up to 1M for 10+, I'd have payment bands for every 10 games he plays up to 50 appearances and then a 1 or 2M payout if they stay in the league.
That's the sort of deal that makes sense to me. I could see 8M upfront with 4M in add-ons. It all depends how much they really want him, we have leverage simply because we can choose to keep him but we also don't want to scare them off.
Throw in a decent sell on fee and a total fee of 12-15M is good, basically, the sell-on fee is schmuck insurance in case he turns into something useful in the league.