There's always more costs than the most obvious ones, so when I price something, whether it's under $5 or much higher, I always consider store subscription, PayPal fees, eBay fees, stock cost (including a percentage for shrinkage - be it loss, replacements, faulty items), packaging costs, running costs (that would be net access and hardware) and so on, and then arrive at a price that will cover those - sometimes individually, sometimes collectively, as well as allow a little for re-investment. For these items, I also look for things that no one else is generally selling so that I don't have to compete (price-wise). 

I have higher priced items, so I don't often have to go to special effort or expense to post a $3 item.

The lower priced items also help maintain cashflow, which I have found to be just as important as profit percentages, because they're currently allowing me to expand a little more rapidly than I have been before I introduced them.