The majority of wallets sold online are fake. The websites that sell them are almost 100% fake. I do not recommend buying wallet.dat files at all. However, I know there are people online who have purchased wallet.dat from a website and successfully cracked. I don’t remember where he bought it from, but I believe it was about 50 BTC. My advice is to see where your buying the wallet from and see if it’s trustworthy. See if it’s possible to get the public key from the wallet and be sure the seller can answer your questions well.