Well, maybe. If you have the tools, the skills to use them, the time to do the work, the space to do the work, tolerant wives, girlfriends and neighbors, no kids, and the means or physical ability to hump around a chassis to the galvanizer's assuming there is one near by. Not everyone is like you in that regard. There is no question that it is easier to just have it delivered all done, ready to bolt up. Also there is no question whether you will have rot you didn't see or treat inside the frame rail. Not having to deal with refurbishing the chassis will always leapfrog the project ahead. In the grand scheme of things that may easily worth the three large for the chassis. It's called "spending wisely"
You are mistakenly not including the expense of your own time and effort, which is a substantial amount. If it takes you twenty hours of work to get the chassis done and you consider your time at a very conservative $40 an hour, plus the supplies, wear and tear on tools and other consumables...you're pretty much there.
What if you had spent all that chassis time on other areas that show more like the interior or painting the body?
Don't attack with broad statements that only apply to your own particular situation. If spending money on a new chassis makes sense for another person they should not be ridiculed or lambasted for that decision nor should any of us who offer that option as a suggestion.