This can be a bad trap.
The customer wants old code understood and new GUI written in 80 hors. 10 normal business days. I am pretty sure they'll want all sorts of bells, whistles and gewgawas on it. I've been caught in this sort of trap a couple times when I worked back in Russia - bad experience for everyone involved. The customer wants a project done on the cheap, I think they grossly underestimated the cost.