As Devs stated, as expensive as they are, the better quality heaters are truly worth the money. They will last much longer - longer than 2-3 of the cheaper heaters. So they usually end up being "cheaper" in the long run.
I like the titanium fully submersible ones with the thermostat that is outside the tank. These have no glass to break.
Having a heater available for a tank can come in very handy - for many treatments for ill fish are dramatically improved with heat. Dramatically changing temperatures can occasionally cause problems in fish - as in an outdoor area where the temp swings night and day, and different temps can be used to prevent or cause breeding behavior.
A "loose" current in a tank is not deadly to the fish until you stick your hand or something else into the tank and complete the circuit. You are lucky.
If you have not read this article about grounding your tank, it is a good idea that you give it some thought. It is not hard or expensive and can be a life saver.
http://www.kokosgoldfish.invisionzone.com/...showtopic=41152