the absolute lowest I would go for something I was not also producing is $600/day plus $800/week bay rental. for a documentary, i would estimate to be cutting one day for every finished minute of the piece. so working five days a week, I would estimate (as an ideal) four and a half months of editorial and a total bill around $70,000.
so apply that to your own model. the "one day for every finished minute" thing is a good, safe (some might say overly so) starting point for determining how long a project will take. obviously, the real world limits on budget and time will work against that.
it's really hard to say without knowing more about the project. it could be anywhere from the 3 weeks mcgee suggested as a minimum to the 18 weeks I suggested. 40 tapes doesn't sound like a lot though. so you answer lies somewhere in between.
600$ a day + edit bay rental sounds good. BUT, why not use this to purchase your own 10-15,000$ Edit Bay (Avid/FCP/Permiere studio, Turnkey system w/ 3-4 terabyte drives, and a ONE BEAST of a computer), and charge them the rental fee, and pay for your own machine, AND make some extra dough off it. Then after wards, you can be prepared to take on Big projects as they come.