A few other options are (tho these are not necessarily cheap, that's been covered by those already mentioned...):
Warmoth, Not necessarily "cheap" but very good and reasonably priced for what you get. They sell body blanks already routed for different parts if you want (completely un-routed of course also...). One of the most critical points of a bold on guitar is the neck pocket (where the neck connects and is bolted in) so getting a squarish body blank with this pocked already routed to perfection will go a long way to a sucessful project. They sell a full line of necks and you can customize one if you want something specific, including blank, unshapted headstocks if you want to shape your own. You can also purchase the bodys in various standard shapes, and various hardware routing, and any combination of them, depending on what you want to try yourself. They also sell a great range of hardware and electronics... Plus, just about everything they sell, can be worked to whatever degree you want, from a wood block, to a shaped, routed, and painted body, and they have a HUGE array of options and customizations from body woods to custom exotic tops, bindings, fret markers, etc...
Carvin sells guitar kits that are also top notch (and probably a good bit pricier than those mentioned by others here), and they sell "Neck-Through" necks, which eliminate the need to route a neck pocket and bolt the neck to the body. Instead, you glue "wings" onto the side of the neck blank, which extends the entire length of the body. Then you can shape the body and route out for the hardware and electronics to meet your needs.
