It's a strange one as I struggle to actually think what is an acceptable value for him, especially when I take the 'Blue Tinted' glasses.
The easiest option is to compare against other players and their values, yet as yourself and
@Rook mentioned, this isn't an perfect option.
Kevin De Bruyne left Chelsea for £18m and I'd argue he's a better (or consistent) player than Mirallas. So he must be worth less than £18m?
Moreover, there is the issue that his contract is expiring and his current sulking persona, so does that also mean that his value is lower?
But then he's a proven Premiership player with international experience and does have talent and ability. Some would argue that adds values?
Really, it depends on the current market value of players of his type and stature at the time, who he's going to and how much they value him at.
Personally, I think the £18m some people are spouting is overly ambitious, but it's not impossible. Especially as the TV money will inflate prices.
I'd suggest his true value is more about the £10-13m mark, however if people really him then his value could easily be manipulated higher.
Yet equally, his actions and those of the club as a whole (our ambitions and other transfer activity) could easily force it lower.
Any less would be poor business and £15m I would be very happy; £18m I would be excstatic. Anything over then we've used a SWAG bag.