The fact that we signed Benni for only £2.5m (reportedly) is irrespective with regards to a selling price. Not even sure why I'm writing this as until its obvious he's leaving as I'd like to think he'll stay. Anyway...
...the selling price surely has to be relative to his worth to the club, not from what we paid for him. We paid £3.3m for Shearer and sold him for £15m. We didnt pay anything for Duff (as far as I'm aware) and sold him for £17m. These kind of players added that extra quality to the Rovers team and it was a loss when they left, not just because we lost our idols but they could change games in our favour. Losing players like that comes at a cost to us so we should be rightly reimbursed with a suitable transfer fee.
Benni has scored 18 Premiership goals in his debut season and although he comes across as a lazy git every now and again, he has still got those goals and proved he can cut it in this league. His age is the only thing that should have any affect on his fee as he's no spritely 21 year old anymore but he's a competent 29 year old and I think he'll score similar goals next year if he stays and plays the 50 odd games he did last year. He wont get that number of appearances at the big four if he chooses to leave.
If he's going to leave, and I'd want him to ask so we dont have to pay a "loyalty" fee, I'd be looking between £12-15m and anything more than that would be a bonus.