What would be my best bet for a non-expensive introduction into this world?
I've no idea what you plan on using the back for but all this stuff is very cost intensive. It's not just that digital backs, it's the whole rat tail that goes along with it.
With a MF back you really have to ask yourself: "What is my time worth?". If you're shooting as a professional your time is probably worth enough to justify the investment, if you invest right. That means a rock solid system that works in your shooting environment.
You have to think about the speed which the back lets you work. You have to think about how you're going to shoot on location. How big of a file do you need? Do you want great skintones or sharp stills? Etc., etc.
And then there's the software...just read some of the threads here and you'll get an idea what that means.
IMO, Your best bet on a small budget is first getting a dealer or two to let you try out the different systems, then renting and learning first hand what the real costs are (the rest of the camera, the computers, the software - Read: the time investment) and finding out exactly what you need. Find out what a digital back can do and what it sucks at. How it works with your camera, how the file looks, how the software works.
A used Leaf Aptus 17 or 22 is a fine piece of equipment for a good price and can make a great file, but IMO it's better to spend $1000 dollars on rental fees and make a wise desicion than to "save" a thousand and have a 6, 7, 8, 9, 10000 dollar nightmare on your hands.