Sorry the 15m
I think £15m is the combined cost of the land and construction work.
The actual and current value of the stadium is another issue altogether, though you can bet that Kassam is wanting to recoup all of his original investment if any deal is to be done.
It's a tricky one, hence the 1/2 year timescale for a deal to be thrashed out.
£15m for a 12,000 doesn't seem too bad when Brighton are shelling out £93m for a 22,000.