By my calcs, Thuram will cost us the same between gross salary and amortization than what Rabiot was costing us. The reason why Rabiot wants a much bigger salary is partly down to tax by the way. He was a beneficiary of the special tax treatment to lure players from foreign leagues (Ronaldo had this and a few others). Meloni scrapped this scheme. It means that Rabiot's average tax rate goes from 24% to 46% - meaning his net salary goes from 7m to 5m. The fact that he was asking, what 8-9m? That would mean our gross payment to him would go from 9.2m to ~15.5m.
By comparison, K. Thuram is currently only getting paid 0.5m net at Nice. If we paid him 2m net, it would quadruple his salary and his gross salary would therefore be 3.7m. Transfermkt values him at 35m - but with only 1 year left on his contract, you would think we could get him sub 30m. Assuming 30m, he would cost us 6m a year in amortization - so his total cost would be equal to what Rabiot used to cost us and he is still 23 years old.
So in theory, if we caved to his demands, it would be equivalent to buying 2 K Thurams.
Not sure how relevant this is, but even the great Luciano Moggi rates him highly.
However, in order to make a financially sustainable transfer market you need to make sure your player costs in gross terms doesn't change much and you can add the many plusvulenzas we are making. We will easily turn a big profit if we sell the likes of Soule, Iling and Barrenechea for instance. The biggest issue though is cash. At the December half we only had cash of 25m and debt of 330-340m (or net debt of around 305-315m) against equity of only 74m. I can make the numbers work easily on the P&L with the transfers we are making - but Juve will need some form of cash injection because many of the players we are offloading are not netting us much cash against what we intend to spend on new player transfer fees - unless we can spread the payments over many years.