That is a really difficult question to answer. It's not just the machine itself, but all the technology behind it that's made Makerbot possible, and the 100s of machines that have gone before it that evolved into a modern Makerbot.
For example the Arduino that drives the whole thing is a masterpiece of modern semiconductor technology. Quite possibly thousands of man-years have gone into developing that board alone, and that's on top of the millions of man-years required to build the semi-conductor industry.
And in terms of software, the g-code language must be at least 4 decades old by now. It started with subtractive processes (e.g. milling) but additive processes like Makerbot uses aren't all that new. They've been doing it with powders since the 70s. There have maybe been 1000s of people involved over multiple decades to create those processes. Skeinforge is a miracle in itself.
Think of the Makerbot as representing the peak of modern technology. All of the industries such as semi-conductors, micro-computing, software, mechanics, electronics, materials… all focussed into a single product that is so cheap and effective that you can build one at home.
My best guess? 10s of millions of man-years are required to get today's 3D printers, if you had to start from nothing. Makerbot has stood on the shoulders of giants, as do we all.
But if you mean just the Makerbot itself, ignoring all the off-the-shelf products they leveraged off to "jump-start" their development? Probably two guys for 12 months :-D
