You are using the word controls which can be interperated different ways.

If you mean the piece you hold in your hand then you mean the Transmitter. These as a rule have to be bought as the amount of work required to produce one would be immense and also you would have to get it CE compliant. So people buy them. They are available from http://www.technobots.co.ukwww.technobots.co.uk . As for whether or not there cheep that depends on how much money you have. Something which is 5pounds is expensive if you have no money. You can expect to pay around 100pounds for a 4channel transmitter and receiver.

You could mean what is used to control the motors. These are called speed controlers. These can be made or can be bought. Making them is very difficult and should not be attempted unless you have experience with electronics. They can be bought again from http://www.technobots.co.ukwww.technobots.co.uk There electronise range is suitable, you want to use 4 drills so use the 30amp ones. The other choice is the IBC from http://www.featherweights.orgwww.featherweights.org These are much more expensive but an all round better controler. Electronises are around 40pounds each but you need 2. IBC is around 180pounds but you only need one.

Crystals cost about 12pounds for a set of 1 transmitter and 1 receiver crystal.