I'll throw my 3 cents from some personal experiences I've heard about.
I think you're running some risk selling something you developed in the Navy if they own copyrighted stuff as well as patentable stuff. On the other hand, their business is defense; do they really spend a lot of time hunting down inventors profiteering off ideas that may have been thought up during Navy time?
I largely agree with the view that the cost is rarely worthwhile, especially if you're a lone inventor. If you're friends with a lawyer, you might be able to get a patent for less than $4k, but they can easily go north of $30k. Supposing you do poney up the money, that doesn't mean you're entirely safe. While you're trying to ramp up a company and short on cash, a bigger company can come in and sue you regardless of how valid their case is. These cases can take years and hundreds of thousands of dollars, and in most cases, you'll run out of money before the attacker does. What's worse, if you have a "closed-source" idea, the company can claim that you're infringing to force you to reveal details of your design. You'd have to open up to prove otherwise. Finally, big companies like IBM with treasure chests full of patents can basically claim infringement by anyone in the world. They use patents as a defense strategy to keep other companies from suing them. Summary: it's very easy for a big company to obliterate a small company, patent or no patent.
So if you do get a patent, I'd recommend trying to license it to an established company that can play ball. I don't think it's true that companies won't even meet with you out of fear that you'll sue them for taking their idea. There are nda's and such that help mitigate that concern, and furthermore, a large company has little to fear from an individual.
Either way, a provisional costs only $100 or so and let's you say "patent pending"--and this could make a competitor pause long enough to give you time to come to market.
All and all, unless you have something with mass-market appeal, it's probably not worth more than the provisional.
The philosophy I've gone with for my company is to develop things quickly rather than try to protect something. Most of our products won't be patentable, and even if they were, there's little chance the ~20 year protection would help when the hobbyist market is completely different in 5 years.
I'd love to hear opposing views. My anti-patent stance is definitely biased by the market I'm in.