Hi Dana! What a nice gift! My son and husband had a blast building a car at the Downtown Disney Ridemakerz earlier this year; it's definitely a fun store for boys, both big and small!
The price to build a car can vary widely, and the final cost depends on how much customization you want to do. At RIdemakerz, you first choose a base car, and then you customize it with wheels, sounds, stickers and more. When we visited Ridemakerz, you could choose a base car for as little as $10, with the prices ranging up to almost $40. The base price includes wheels, rims, and one sound. You could choose to upgrade those items for an additional charge. There were free stickers you could use to decorate your car, or you could purchase stickers, spoilers, mufflers, lights, etc. If you want to drive your car by radio remote-control, that feature costs about $25, plus the cost of batteries for the remote and the car.
My son chose a $15 car and stuck with the stock wheels, hubs, and sound. He added remote control and purchased the rechargeable battery along with a spoiler and a few other items to customize his car, and our total cost was around $65. In my opinion, it would be hard to get out of the store spending much less than that, unless you don't get the remote control option, but that's really the feature the boys seem to love the most. If you really want a souped-up car, you could easily spend $100.
Hope this helps! Have a magical vacation, and come visit us again at the Moms Panel if you have more questions!