Cheap, from $30 to $50 and you can usually get the place you buy the MDF from to do the cuts for you so it's just piecing them together like a puzzle. Get Win ISD its a program for designing sub enclosures, look it up on Google and match up your Kickers model numbers from the subs. This will tell you how big of a box and how big of ports you will need. Good luck on this.
ruben there is actually one place i know of that will make a nice box for you. try a place called "discount stereos" in the city of norwalk. i seen them make boxes for customers and they do a decent job. that is if you are not comfortable with making your own.