Gerbs,
I cannot speak for the rest of the "big" rocketeers out there, but can tell you how I do it and what I have seen.
First, having some extra cash is a plus because, as you noted, many of these rockets are over $1000 just in parts, not to include a similar amount for propellant. This income could come from a good job and low expenses, or from spreading you building over a year's time. You could also just be wealthy like the Gates brothers, who actually founded and owned Xircom, a VERY large, successful network card company in the 90's, which they ultimately sold for millions to Intel. They have since retired at a young age as millionares and build big friggin rockets...now THAT would be the ultimate in jobs/retirement plans!
Also note the rate of construction...even the biggest builders rarely construct more than a couple ships a year, while smaller rocket builders and fliers may do dozens or even hundreds of models/kits. The same goes for flights...I rarely see Level 3 fliers launch much during the year...they usually come out for the opening launch of a season and may have a few launches per year, but not much more; there are exceptions, of course, and they are the ones you may see more often. Some save for LDRS and unload all they have there, others spread it out over the season.
Moreover, remember most have already purchased electronics and are used to scratch building and glassing, thus they save alot of money by preparing/making their own components ahead of time. As far as propellant goes, you will see a few sponsors for large projects, but many save up for what they need and buy their propellant across the season.
Well, that's just my idea on how it is done...
Cya,
Carl
L3 Flier