Best cheap 3d printer ?

Initial cost.
Long term cost (materials, maintenance, parts, repairs...)
What material(s) will the printer work with
What is the resolution of the printer in each axis?
What is the rate of material deposition?
Build size limits?
What file formats does the printer accept?
Is the slicing software for the printer any good? Can third party software drive it?
What technology is used (FDM, SLA, SLS, LOM, DLP, DMLS...)
What are the cleanup requirements once a part is removed from the machine?
How much power does the machine use?
How difficult is maintenance? How often must it occur?

Many of the requirements will be driven be the expectation for the printed parts. Determine what will be expected from the printed parts, then buy a machine which can make them.
No 3D printer can meet the requirements for every part. The best you can hope for with a single printer is to at least be able to make some of the parts you want.

It is not cheap, but if I were going to buy a 3D printer, I'd look at the offerings from Formlabs. There are some other systems which utilize wax as the printed material, those look cool as well. But, everyone will have different requirements. My main goal would be small/medium parts with very high detail. Something 3D printing is ideal for. Someone wanting medium/large parts with low detail might be better off with CNC equipment.

Check out 3dmake.in

Literally, there are lots and lots of parameters to consider when buying a 3D printer and some of these are as follows:
• Budget (highly important)
• Type of 3D printing technology used
• DIY 3D printer or fully assembled one
• Supported printing software
• Supported printing material
• Personal or professional use
• Printing resolution
• Printing Speed
• Heated bed or not
• Auto-leveling bed or not
Besides this, there are lots of others and you can get a better view of these when you visit a good 3D printer store such as ‘3D Printers Bay’.