From what I understand growing a laser diode is a lot like cooking meth. Sometimes you get a nice, perfect crystal.. sometimes you get a flawed one. So they separate out the diodes into good ones that become high power diodes and bad ones that get powered lower.
Now, if I'm right about all that (and someone who knows more about diode manufacturing may have something to say) then your cheep 5mw greenies would be made out of the "bottom of the bin" diodes that could barely even manage to lase at all. So one might work great while another from the same source might peter out after two hours.
If you really want a cheap green diode, I suggest you buy two. That way you have a pretty good chance of one of them working.. and buying an extra probably won't cost as much as shipping, so it's like "buy one get one half off."