Forts and mercs will use up money fast. You can scrap the fort later and get an infusion of emergency resources if you need it.
If my cash is like 800k or above I'll build mercs, and stop when it gets down to a few hundred K. Who cares how much they cost? Mercs are much more cost effective than wasting unused cash. If you don't play really fast and aggressive you are probably gonna make use of mercs, since you'll probably hit resource cap a lot. Don't be scared to use them, either. You're never gonna "get your money's worth" out of mercs, priced as they are. Just think of them as kind of bonus ships that make your fleet a bit scarier. Or, hoard them like kuresuti does until you have a significant force of them.
And obviously if you have zenith traders you can throw down all sorts of goodies, pause them, and continue building when you have too much cash.