It isn't that cheap, and it isn't exactly easy. However, it has been done many many times, so you are not breaking new ground yourself.
Here is an article that talks about some of the stuff that you will need to do, and stuff you will want to think about
:
Pelican Parts Official Guide to the 914-6 Conversion
Here is the Pelican catalog section pertaining to the conversion stuff:
PelicanParts.com - Porsche 914 (1970-76) - 914-6 Conversion
There are a lot of ways to do a Six conversion. There are a number of other things that people often do along with the conversion that are not strictly necessary, but are often a good idea. (E.g., five-bolt wheels, 911 brakes, extra oil cooling, etc.) If you are patient about waiting for good deals, a good scrounger, and do most or all of the work yourself, a minimal sort of conversion can be done for $5K or so if you have a running motor complete with carbs.
Prices quickly escalate if you need parts NOW, or if you pay someone else to do the work, or if your conversion is better than the bare minimum. I keep hearing that a "reasonable" conversion would be in the $10K range for the parts (without motor), and when you add labor costs it gets expensive very quickly.
That said, a 914-6 is an awesome car. As Cairo said, the sound of a flat-six right behind you is addictive, and the power is even more so...
--DD