My rating scale is similar to the ones used by Spill/Double Toasted and Jeremy Jahns. Instead of a score I use a phrase that generally summarizes my feeling towards the book.
Vintage: A great comic that you should get no matter how expensive.
Full Price: A good comic that you wouldn’t feel bad if you bought it new.
Buy for a bargain: An alright comic that if you can get it cheap, by all means go ahead.
Borrow from a mate: A bad comic that you should only read if it’s free.
Rubbish: Just an absolute failure of a comic, avoid at all costs.