I always charge a minimum of four feet for a frame no matter what the size. Smaller frames are often more difficult to work with than larger sizes.
OK, I do have an exception. If I'm using a very expensive moulding and have a "scrap" in stock, then my customer's attitude sometimes will dictate my willingness to budge.
Pricing is very much a science and an art. I often get jobs which are fixing up someone's old frames and rejoining, refitting, re-matting, remounting, re-hinging, re-la-de-da-de-da-ing. I actually enjoy this type of work and dread it at the same time. The only way to do this type of job is to guesstimate the materials and time it will take to do the work. I take it in as a "labor-of-love" and do everything I can to preserve the providence of the frame and the work enclosed...including reapplying other framers stickers, copying information I find inside the frame package, upgrading everything I can to conservation quality.
New frame pricing, unless very customized, is fairly standard as shown in another thread now on this forum. Sure there are aberrations, but most prices fall into a close spectrum range. Differences can be explained by geographic locations and resultant overhead. Even though the formulas used for pricing vary greatly, the results tend to reflect the free market.
One thing to note, however, is that even though the prices are usually similar, it is the minute differences that reflect profit. A 5% difference may not seem large, but the difference over an accounting period on the bottom line is humongous!
I guess my best advise would be to price your work at what you feel the market will accept...and then add 10% more. Deliver quality work and the customer will generally never complain. That has been my experience.
Dave Makielski