I'm one of the biggest corner cutters and cheapskates around, So I often buy the cheapest thing I can find. Metal detecting and socket wrench sets are about the only things that I have REALLY learned the hard way not to skimp on.
Again, I caution that I'm speaking from my knowledge from ten or more years ago, and I don't know how much the technology has changed.
That Said:
Even the cheaper units can be pretty precise... The problem is, communicating that information to the operator;
The cheap units communicate with squeaks, squaks and squeals, and maybe if you spend a little extra an analog or pseudo-digital meter that swings a bit when you pass over something metal. Translating those noises takes a LOT of practice and good hearing, as the difference between a nickle plated bumper bolt and a gold dubloon can be pretty subtle.
The better units still squeak and squawk, but have digital meters that narrow down what your machine is sensing plus give you an estimate of how deeply it is buried. I'd say the old Whites Eagle digital display was at least 85% accurate in indicating the classification of the target and the depth.
With My old Whites Classic II (squeak and squak only) I was never able to be more than 25% certain of what it was sensing and 10% correct at guessing the depth. Some of the 'old timers' that had been using one or one similar just about every day for years could do some amazing things with a cheapie detector, but they had invested hundreds or thousands of hours of time seriously learning how to translate those squeaks and squawks.
Back then, the most respected manufacturers were Whites, Garrett, Bounty Hunter and maybe Tesoro.... I say maybe Tesoro because they didn't have a lot of users back then, but their factory was about 5 blocks from my house, and I stopped by there a few times and they impressed me with their seriousness about building good machines at a good price. I never actually used one though.
Minelab made some that were THE choice for serious gold prospectors, and for underwater, the Fishers were popular.
Whites is my personal preference and their top-of the line models do EVERYTHING real well.
Not sure what your local Radio Shack has on the walls, as some stores can carry whatever they want in addition to Radio Shack branded items. But I checked out the Radio Shack website... The only one I would consider there is the Discovery 3300. They call it a Bounty Hunter, but it isn't really... The Bounty Hunter website is all screwed up, but best I can tell, they don't offer a Discovery 3300... I think they probably manufacture them for Radio Shack, but are selling them for less there because they don't pass the Quality Control requirements to be part of the Bounty Hunter branded line.... Kind of like Mitsubishi sold their QC Reject TV's under the Memorex name back in the 80s...
Or perhaps Bounty Hunter did the design work and farmed the manufacture to China or Honduras or something.
Well, good luck.