This charge varies from registrar to registrar. Insofar as a .com is concerned it should be renewed before expiry however there is usually a grace period of up to 40 days allowing renewal at the normal rate and then it goes ino the redemption period of usually 30 days or so.
If you then decide you do want to renew it at this point it has to be restored. This is, I believe, a manual process whereby the registrar has to submit actual paperwork to the central registry and most of the charges from the registrar are paid to the central registry.
Lesson #1 - Don't forget to renew on time!
I've seen lots of different prices bandied about from USD 50.00 up to USD 125.00 or so.
Obviously you do risk losing a popular name if you let it drop but if no one else wants it maybe letting it drop and renew would be cheaper. It all depends just how important or not the name is to you?