Wanyama cost Southampton 12.5mill when he had 2 years left on his contract at Celtic. Now add 3 years of good premiership performances and experience, factor in the percieved relationship between the 2 clubs, how other clubs are interested in him(eg. Leicester), the rise in transfer prices, the rise in TV money and that it's a move to a PL rival, and I highly doubt he'll be a price below £15mill. Personally I think it'll be closer to £20mill