...but "cheap" is relative.
I have a friend that has used a Fisher 1280 for decades and has MUCH loot to show for his efforts. They're a good machine. Sure, they won't find everything, but let me tell you, the Fisher unit is a good, good unit. You can look on ebay and find good deals on them form time to time or just save up and pay the $600+ for the machine. One gets what one pays for, you know.
Although I would not recommend buying a Viper or any other Chinese machine (they don't back their machines when something goes on the blink, I hear), perhaps it is better than not being in the water at all. But before I bought one, I would give some serious thought to waterproofing an ACE 250. There's plenty of folks that have gone down that road using a Pelican or Otter box. That kind of a set up would be good for chest-high wading...at least. And...you would be in the water.
I finally bit the bullet and bought a new Excal II last year and have never regretted it. I saved for a long time and just did it. But until then, buy the best you can afford and have fun with this hobby. Lootin' swimmin' holes is the way to go and I likes it!!!
Blessin's to you. Let us know what you wind up doin' and be sure and post those finds that are in store for you in the upcoming season. I've been doin' this gig since 1970 and I have learned a couple of rules of thumb: (1) Buy what you can afford and (2) get in the field and USE the fool out of it. Be persistent, do your homework and go work your plan. No matter what machine you see on the beach or lake, NO ONE can find it all. So, go get 'em!
aj