Opening paint shop, how to pirce jobs.

Hey from all the responses I ahve gotten from you guys about my work over the last couple of months I have decided to open a a pintshop up here in Canada. LayedOutPaintwerks.Just wanna know, how do the rest of you painters go about pricing jobs out in advance, like do you figure out the hours, or through complexity or what, I find it really hard to give a good idea before I paint something, ya know before I figure out the hours involved.