I don't think pricing things by the square inch is workable because, what if you don't paint large? What if your large paintings are not as detailed as your smallest works?
I can see it being used as a baseline, but never as a barometer.
So many other things, IMO, need to be taken into account, like the artist's resume (exhibition history--where
they have exhibited, who they have exhibited with, what collections they are part of, awards they have earned, how long they have been painting, etc.). Also, perhaps who they have studied with and their education. I say that even though I am self-taught because it should be taken into account if a person has earned their MFA in painting. That shows something substantial. You don't just pay for an education. You have to earn it too.
Also, the artist might have a bibliography--critical articles that have been published or written about their work. That counts for a lot too. A history of sales is also important.
Pricing your work for retail is tricky. My personal opinion is to always price your work in retail prices on your website so that you are not undercutting an gallery you wind up with. A gallery will usually double the wholesale price (what you want for your work). So the market value is different from the price YOU want. You have to keep this in mind. You should also keep in mind that if you price too high, you can shoot yourself in the foot for a long time because dropping your prices later (after someone has already paid for something at that cost) will denigrate their investment in you. You can always raise prices, not necessarily drop them. Not without consequence.
You should also look at what peers are pricing their work--peers with equal credentials, or thereabouts. Peers showing in equivalent galleries or shows, etc. Something along that line. You don't want to come in too low or too high.
Never undervalue your work because then so will your buyer. If you're with a gallery, the price-setting should be a collaborative decision. Look at your gallery like your spouse and the art as your children. If you are feeling uneasy with your spouse and are having thoughts of leaving them, it might not be a good idea to get married in the first place. You should have a happy marriage and feel comfortable, and trust them, just like any relationship. And it goes both ways. Communication is key. Etc.
Hope this stuff helps.