I still don't think that "doors for desks" is the right way to treat your employees, no matter how bargain-crazy your vision.
For legs you can use some combination of: shelving, filing cabinets, sawhorses, brackets+lumber, and a-la-carte legs.
At my parent's software startup they used doors for desks for their whole 20-year run. Be careful though -- with great surface area comes great responsibility! Once my mom's desk clutter reached critical mass, it stayed that way for 15 years (18" deep in the back, 2" in the front).
* http://glinden.blogspot.com/2006/01/early-amazon-door-desks....
* http://www.fastcompany.com/magazine/85/bezos_2.html
It is questionable whether in the age of IKEA, the door desk is still truly frugal, but it is a very tangible reminder of the principle. Very tangible, very sturdy.