If you can find a cheap route into MF digital then I, for one, would be very interested in knowing what it is!! Seriously though, I guess it depends on what you are looking to achieve from MF digital.
Cheapest new: Mamiya 645AFD & Leaf 6mp - $6999
Other: pre-owned back & camera platform - eBay is active with backs and kits at the moment, e.g. last week alone there was a Hassy 555ELD & Kodak Pro Back go for $7600, I bought a 2nd Kodak 645M for $5100, Robert White in the UK have an ex-demo (basically new) 645M for $7000 or so, and I've also seen Phase One H10's & Leaf backs available too.
This was my personal journey:
I wanted the fabulous colour capabilities of (what was then) a high MP platform that I could use with great glass, a rugged and usable camera and something that wouldn't cause me to re=mortgage the house. Being a big chap I prefer the form and function of MF vs 35mm - I guess it's a personal thing. I went with a Kodak 645M and Mamiya 645AFD setup and saved a little by getting ex-demo rather than new. Since then of course Kodak have dropped out of the back market so that was bit of a dead end as far as future-proofing the back is concerned. However, to be honest with you I don't see this as a problem as I'm delighted with the quality of the results compared to anything I've used before (D1, D1X, Leica D2, Sony F828, Minolta A2, and FILM!!! etc.) and the 4k square image format suits me perfectly - I just wish Mamiya would deliver on their promised super-wide but that's a different issue ...
With hindsight I might have gone for a Kodak Pro Back Plus instead of the dedicated 645M as this would provide more flexibility as far as LF, MF and specialist cameras & lenses are concerned.
This raises the issue as to whether you select a platform and then explore the digital solutions that work with it or whether you decide on a digital back platform and look at the camera systems that support that back.
I think the cheapest way right now is to either buy something like the Leaf/Mamiya 6MP package which runs $6999 I believe new, or alternatively run the gauntlet of eBay or dealers and pick up a pre-owned back such as a Kodak, Leaf, Phase One, Imacon etc, and the appropriate camera platform. In terms of platform you are looking at Hasselblad, Contax, Mamiya, and then depending on the back you are either limited to contemporary models such as H1, 645AFD or you are looking at things like 555ELD, RZ, 645 Pro, 5xx series and so on.
In summary, I'd find a camera platform that you are prepared to invest in and then find the appropriate digital back to suit it. So long as the camera platform looks like it is going to be around for a while and isn't going to end up being discontinued you should be relatively safe. I thought the same about Kodak, but hey, who knew?
A word of warning: In my experience owning a digital back is comparable to owning an exotic car - i.e. make sure that you have the funds to maintain and fix it if necessary. I had a slight intermittent problem that occurred out of warranty (don't they always?) and you wouldn't believe how much spare parts for these things cost!! (It worked out cheaper to live with the problem and just get another pre-owned back instead). The other thing, although I mentioned eBay as a source for well maintained equipment if you don't want to go retail, it is a nasty shark infested place with no end of scammers, fraudsters and irritating people who can drive you to distraction. Once they sense that there are expensive items for sale and the opportunity to try to grab large $$$ in a fraud, you'll be inundated with quite laughable attempts to scam you. Be careful. I think I could write a book on eBay fraud attempt identification now.
Alternatively, crack open the piggy bank and spend $25k+ on an Imacon/H1 deal, Phase One P20/P25 etc ... too much for me.
Hope this helps ...[/font]