Hard to know what to tell you without specifics like how the house is constructed (what type of walls, floors, roofing, size of current pipes and there condition) and what part of the country you are located in. I think both estimates sound a bit high, but with out knowing the previously stated variables I can't help you. If I had to take a wild guess, I would tend to agree with the last 3 responses. The Achilles heal of the duraflow system is that if your pipes are bad enough to need repiping or coating, there is a good possibility that you will have a least one break when they blow the abrasives thru the lines. Those leaks usually end up being in inaccessible locations. That means you will have to cut or jackhammer open something to fix it and that defeats the whole purpose of pipe coating. In my area, repiping is usually about half the cost of coating, comes with a better warranty, and is very non-invasive. In fact, some of my customers that where not present during the job initially didn't even understand what was done.
Just my .02
P.s. Repiping is my specialty. I've repiped far more than a thousand homes, so my opinion could be considered biased.