The average speed of a radio controlled vehicle varies greatly by the type of radio control vehicle, brand, and frequency of the radio control. It can be super slow or can go super fast.
10 mils
Radio controlled vehicles can go a variety of speeds, much of it depending upon what sort of mechanics are built into the vehicle's engine or battery.
RC Planet, Hobby Tron, Tower Hobbies, Argos, eBay, Amazon, Overstock, Hobby Lobby, Wal Mart, and Target are all places an individual can go in order to purchase a radio or remote controlled vehicle.
You can buy radio-controlled gliders for kids from the Hobby Lobby. On their site, they have gliders and accessories available, as well as other radio-controlled toys.
The fastest Tamiya Radio controlled vehicle would be the RTR Levant car. At almost $600, the RTR Levant car gets the greatest of reviews and sounds like it has plenty of speed, more than some people can handle.
You can purchase a radio controlled truck from the websites HobbyTron, Towerhobbies, NitroRCX, HorizonHobby, Kmart, Target and Amazon. Ebay also has used and new radio controlled trucks for sale.
Radio controlled helicopters on average cost $50. One can purchase radio controlled helicopters from online retailers like eBay, Amazon, Model Zone and many more.
A radio controlled car or R/C car is a powered (electric or gas) model car driven by a radio control system. Inputs from joysticks (or a wheel and a trigger) on a transmitter are sent to the car's onboard receiver. Radio controlled cars can range from simple "stop and go" cars to competitive racing models, which feature all the complexity and adjustability of a full sized racing vehicle.
A radio controlled tank would be controlled like any other type of remote control vehicle, though a tank would be much stronger than a car perhaps. You can find more information here: r-c-tanks.com
There are a couple of websites that sell small boats that are radio controlled in a way a child could use. You can check out the website, Radio Controlled Toys, to purchase.
If you have a Radioshack in your area they sell them. The cost for a radio controlled boat depends on the size, what it can & can't do, and the brand.