Be aware that boxes are the most expensive component of most games.
Are you looking for a one off or production?
Just for prototypes really ... I saw on SPOD that their game boxes were $2.00 and I thought "wow, if I can show off a prototype at an Unpub in a printed box (I do graphics myself, so there's no artist fee) it may look pretty slick and generate some more interest. Problem is, the games I'm working on aren't just cards - there are multiple components and the size offered by SPOD isn't going to be enough.
I figure the bigger the box, the higher the cost.