Well, for a handgun and assuming you and the seller are in different states.
If the seller is a dealer he can ship to your FFL using USPS for around $20 or less easily
If the seller is not an FFL, he will spend $35-50 or even more to ship it legally
Both of course somewhat depend on distances between you and the seller.
Your FFL has to then transfer it to you and chanrges a fee to do it based on the local market and whether or not he sees transfers as taking his sales away or as a way to make money. Some charge as little as $20, others as high as $75 or more.
gunbroker.com and other sites have tools to help you find a transfer FFL near you.