Reality is, you will not end up saving yourself any money...Fact is, kits are all different, how you set up a standard off the shelf kit made for your sled still has lots of variables...and needs to be tuned to you, and your locale...
So, if your ok with tweaking and tuning and trouble shooting all the variables (which can be as simple as clutching, to coolant flow, to oil supply lines, to Cam timing, and head shims etc. etc. then rock and roll go for it...
However, for over 99% of people it is not reasonable to do this...Why? Even standard proven kits (think MCX, MPI, etc.) there are no less than a dozen things that are custom for every sled/rider combination...
Each day I ride is sooo limited, 20+/- rides a year...that the actual cost difference between buying a proven kit with support, is worth every penny versus ruining a day or trip of riding...It is just not worth it...