I've had this sort of thing on a few vehicles and have had it fixed with varying degrees of success.
Sometimes a pro repair will go great. I had some areas like that fixed on the Jag over 15 years ago and they're still fine today. I've had other such rust fixed on other vehicles and it came back
Good shops are hard to find and they aren't cheap. $150 isn't nearly enough for a good fix. I'd try to find somebody *good* who can fix it right and will stand behind his work.
DIY fixes never seem to hold up very well; this stuff is a lot harder than most people think. But if you want to try it again, skip the zinc primer, it never works all that well IME. I *have* had some incredible results with Eastwood's Rust Encapsulator. I did a quick fix on some similar areas on the Blazer: I ground down the rust, applied a rust converter, then painted over that with Rust Encapsulator. Put some touchup paint over the Rust Encapsulator. This was last summer and I didn't expect it to last very long. But here I am 9-10 months later and the repair is still holding up. That's not really very long in the big picture, but hey, it's a year's worth of improvement for not a lot of work or money.
For areas where you can't get to the rust, I'd coat it with Eastwood's Heavy Duty Anti-Rust or, better yet, AutoInt's rustproofing stuff (forget the name, I got some old MOPAR-label stuff from them when they changed packaging for Daimler-Chrysler).
Links:
Eastwood Company Home Page and
Automotive International - Valugard Product Line