There is a big difference between the Toy type metal detectors and serious hobby or professional type detectors. We see a lot of Chinese import detectors that sell for vey cheap prices and other similar looking detectors that sell for $500 and up. With metal detectors you will get what you pay for. A metal detector that is under $100 is most likely a toy that might find a coin at 2 or 3 inches in the backyard but will not be able to be used at the beach as the circuitry cannot cope with minerals in the Sand giving a false target response. The expensive detectors have Discrimination that allows for some junk targets to be ignored by the metal detector and this saves a lot of digging. If you want to detect in the shallow water in the beach then you will need a water proof P.I or BBS type detector such as those made by Whites or Minelab. Toy detectors will most likely stay in the box once you try to use it. The better detectors go far deeper into the ground and can handle the ground mineralisation.
Scan the web for metal detector tests and read the metal detecting forums to get an idea on how the detectors perform. This can save you from buying a sub standard machine. There are also things called long range locators, these are a total scam, they will only locate your Wallet and the only winner is the seller.
also have a look at http://detectormods.com
Research is the key to being a happy customer.