Instructables

what is the cheapest 3d printer and how much is it?

I would like to know 'cause I want one.

The come for as littel as $300 but the build platform is very small. So you should probably consider the size of the objects your likely to produce and base your purchase from there. Do an internet search for 3D printers and you will find plenty of options between $400 and $1000. Often the cheaper ones you have to put together on your own and buy the parts individually. I.E. you can find the plastic bis as a set but you may have to find the hardware, motors, controllers and software from many different sources. Making it a poor choice for anyone who doesn't have some electronic and mechanical background.

So figure out what you want in a 3D printer. Then come back with that list and we can better assist you. Or we can point you at the more commonly known systems like the Reprap.
If you want a good, fully assembled 3D printer I would recommend going for Solidoodle! The cheapest is about $499 and you don't need to do any construction so it saves you some time.

You might want to check out this link: http://www.cartridgesave.co.uk/news/a-roundup-of-3d-printers/. It's a list of 3D printers and 3D printing companies on the market at the moment, you might find some useful information and you can compare the prices of 3D printers.
They vary quite a bit in price. Don't just look for the cheapest. Look for what they do and if it fits in with your needs...

Here's some resources for you to look at in helping you to make a decision.

http://3dprinterhub.com/3d-printer-brands/

http://reviews.cnet.com/3d-printers/makerbot-replicator-3d-printer/4540-33809_7-35266603-4.html

http://www.additive3d.com/3dpr_cht.htm