It is cool when that happens, but it is extremely important to define the scope of work very well when it does. As if the client had a different expectation for the time frame don't be surprised if they then continue bothering you about details of integration of your feature with the rest of their project for a month.
Yeah, also be careful before you say yes; I had a similar opportunity, insanely good money , but when I interviewed a few devs who’d work for them they had a terrible rep for getting out of scope, moving the goalposts and communicating badly.
I worked at a minicomputer manufacture (okay, Prime Computer). Software enhancements large enough to need a proposal always included a section "Non-goals of this change."
This section laid out the things that might sound like they are related but would not be part of the enhancement. For example, a speed-up of the filesystem might have a non-goal of adding new file types.
