Consider the Big Mac Economist theory, based on the theory of purchasing-power parity we can evaluate players. The price of Big Mac in Germany is 3.90 while it's 2.50 in China. Both Dembele and Paulinho cost 15m to buy from Rennes and Tottenham. So applying this theory will result in Dembele cost of 58.5 and Paulinho 37.5. But this is only valid if the variables in the equation are equal which is not. For Dembele, (given his age, talent, marketability...etc) you will need to multiply by 1 or 2 more factors =73.5-88.5 fair price. For Paulinho (given his age and depreciation) his value should be reduced by 1 or 2 factors= 22.5.
Other factors affecting the price, of course, the timing of the deal and other external factors like Neymar's sale, inflated sales of other platers Lukaku...etc.