I don't think that the quality of your product would be much effected using a nice quality poly batt. As far as the fabric substitutions, I would make my purchases at Connecting Threads. Their non-sale price for wide backing is about what you would pay for a yard of quilting cotton at an LQS. A couple times a year they have it on sale for way way less. I stock up. I would also recommend using their blenders for sashing. As long as it fulfils the batting requirement, a looser quilting pattern would be fine. I wouldn't skip the stabilizer, however.
Using the above adjustments, I would price one out. I have to believe that your time is the biggest cost and except for spending less time long arming, you may not save all that much.