ignoring the last post, $.35 sq/ft is a reasonable number. I assume you're trying to use a cost estimation program and its asking your ink cost psf
taking the point of his post, you should come up with a baseline figure for your costs based on the expenses you incur running your shop. the cost of personnel talking on the phone, sweeping up, the power consumption of your cc reader, time spent waiting for a proof, the coffee pot. all of these are things you pay for that aren't able to be figured out easily per job. not to mention wasted weeded vinyl, or not having the right size roll of material so you have to waste a larger roll or somethign to that effect.
the Sign industry is one of the few where putting a 1000% markup on your COGS isn't unrealistic. selling a banner that cost you 8 bucks in materials for 80 is fairly commonplace. if you are bidding to the point where you need an exact number for your ink usage, you're bidding to low an don't want that job.