He is definitely not worth it. I love Dier - but if I were to list the players from hardest to easiest to replace - Walker would be at the bottom, and Dier right above him. Wanyama is a much better DM, and finding a RCB for a fraction of that price would be easy - even in this crazy market.
But, Dier is also a bit of the heart and soul of the team in terms of energy and mentality, and Poch (and Dele) love him. Levy will not sell for less than stupid money - end even then its unlikely - because Spurs don't need to sell him, he is on a long-term contract, and Spurs don't need the money.