if for example to achieve 100GB/s with xdr2 you need only 64bit but 200 traces, and with gddr5 you need 128bit but only 160 traces, the best option is gddr5 because wider xdr2 bus are out of budget
It is not like more traces on the PCB = higher price.
Price of the PCB is given by total surface, number of layers and design/construction class (if that is right term in English, higher design class mean lower width of the traces and insulating gap).
So whatever memory need more traces it doesn't automatically mean higher price and there are probably other more important factors in decision process.