Aren't the Davis Echomasters like 25-80 bucks, depending on which one you get? A nice dinner out for one - or in my case, just a bloody trip to McDonalds with the kids.
I think it was CWorld that did a write up and test on them a few months back. Will have to excuse the exact time of the article, but I have slept since then. I think you can run through their archives and find it. As I recall, they gave the cheapo little Davis a good write-up.
I would be less concerned about the times you are vigilantly watching than the times you are not. 25 bucks? 80 bucks? Well, that is a lot of beer, but in terms of a boat, not much money.
I will contradict myself on one thing which I would like to point out and you mentioned: Freighters, barges, and Sea Rays... it is not as if they will steer around you even if they did see you. They DO own the sea, you know.