what kind of aftermarket efi system like fast, proefi, megasquirt etc are you running on your street car and why?
I'm chose MS for 4 reasons, price, features/capability, flexibility, and the opportunity to learn.
One of the genius things about Megasquirt is you can start with the top of the line unit with all the bangs and whistles, or you can get started very cheap, and still gain quite a bit of functionality and then upgrade later if you want something that the later revisions offer.
The price of a wideband should be included regardless of what you choose, because you should have one period.
After that a MS1 kit on a v3.0 board that you build yourself is about $200. You can get a "stimulator" that is some circuitry that basically pretends to be a car so you can test the computer out is around $45. So well under $300 and you can remove the factory computer and completely take control of your fuel and spark, as well as add some other cool features.
Want to upgrade from MS1 to MS2? Just pop out the MS1 chip, and pop in the MS2 card in it's place. Change a couple of settings and load the MS2 code and you now have a more powerful MS2 with a faster processor and more features (what i currently run). Want some features that are in MS3? Okay, pop out the MS2 card and pop in the MS3 card... change a couple of settings and load MS3 code and again, now you have a more powerful MS3 with an even faster (and dual-core) processor and yet even more features. Then you can even build upon that with MS3x which adds even more channels to control even more things.
So you've basically got 4 stages to choose from, but you can move between them for the cost of the upgraded components. Also, the same software is used for all versions.
I purchased my kit about 2 years ago now, and I don't have a single regret. I have friends with those multi-thousand dollar FAST systems and I can tell you MS is right there with them. In fact, they are scratching their heads with the FAST and because of my experience with MS, I'm right at home setting up a FAST system. Megasquirt is not mickey-mouse, it's legit.