I think the sell-on to 'boro complicates things. What is that - 35%? Shame. You really want more to compensate the sell-on because taking less than £7m is a rip. Without the sell-on, I'd take the £10m because his contract expires next season.
You need really good circumstances to maximise the return on a player. I'm sure we'd all rate Wood as "worth" more than £10m, but a commodity is only ever as valuable as an interested party is willing to pay. Doesn't matter if you're selling cars, potatoes or footballers. Remember that time Swans sold Gylfi for £50m? Much more than most people imagined. He went for his ceiling value because he was at his peak, happy, performing brilliantly, the timing was right.
Wood has been ok. He hasn't hit his peak yet and won't for a few years. So his valuation is based at least partially on potential. One year left on his contract ruins that value, because all he has to do is hold out until the close of this window, and he can force a January move when Swansea decide getting something is better than getting nothing, in which case they'll probably take less than £10m. The mixture of timing and the fact Wood is still young and therefore represents some level of risk as an invetsment means interested parties have the bargaining power. If Swansea hold out for more, they risk upsetting Wood, which in turn leads to diminished performances which would damage his market value come January, not to mention hurting the team's on-field performance in the meantime.
Swansea is a team players will always see as a stepping-stone, so getting good market value on a short contract is hard. That's why I like the Piroe deal so much, and that's why I'd take any reasonable offer for Wood, because playing hardball here has too much potential to backfire. I'd push it a bit to see if I could get enough to cover the sell-on, but I wouldn't push so hard as to queer the deal. The writing's on the wall, he's as good as gone, and the money isn't likely to improve much. Mess this up, and Wood walks for nothing in the summer.