Terry,
A helmet must pass the minimum test. For example the minimum may be something like a rounded spike weighing 2Kg with a point of 2cm radius must not penetrate more than 10 mm when dropped from a height of 50 cm. (figures are an example only not actual requirements) A ** helmet may have a penetration of 8mm (a pass) and a ***** helmet 3mm (still a pass). In these politically correct days, a pass is a pass and a fail is some improvement needed..
When it comes down to it, if it meets the standard then its safe to wear. Next comes the fit, a thousand dollar hat that is loose im spots and squeezes in others (ie doesnt fit right) is no good for you as it will move around under severe impact, and it will distract you while riding due to pain. A cheaper helmet that fits perfectly would be much better.
The best bet when buying a helmet is to find the one that fits the best then look at the price.
I personaly wont wear a second hand helmet sourced from who knows where ( like cash converters) as you dont know what stresses that it may have been subject to (and what nasty bugs may have lived in his/her head)