In most cases, both sites more or less agree on the player values, but there are some discrepancies. I'd tend to agree more with CIES on the value of Montoya and Ferran Torres and more with Transfermarkt on the value of Zaza and Gaya. The values of Soler, Mina and Maksimovich are somewhat in between the two.
The other take away from all this is that Valencia really can't expect to get much from players already loaned out. Cancelo is the main one that the club hopes to sell, but his 32 million buy option for Inter looks too high and the 20 million value looks more reasonable. For the rest, we are basically looking at loose change in relative terms. Nani could possibly fetch more if he agreed to go somewhere like China. He's rejected that idea before, but if a club came in with a crazy salary offer, it's doable.