Keep in mind that custom built stretcher bars will almost always be the non-adjustable type, put together like a regular, rigid picture frame rather than with the wedge-adjustable corners you get with most standard sized bars. With the fixed bars the only long-term way to tighten a sagging canvas is for you to re-stretch and re-staple, whereas with the adjustable bars even your client can do the job quickly and easily. That's part of the reason I prefer mounted canvases.
One the of the many ways to price your work is by area, + a constant fee. For instance, $0.30 per square inch, plus a constant $100 regardless of size. If you are selling to price sensitive retail markets in multiple venues, that will lend consistency to your pricing and keep all your vendors feeling they are on a level playing field, which is important. It also allows your vendors to quote custom sized prices at the point of sale without having to track you down on the phone. The "constant" part keeps you from being killed on small pieces, and it encourages your buyers to think big. This runs against the meme of pricing your pieces individually by some abstract judgement of individual merit, which is perhaps a better plan if you are primarily interested in selling through galleries that are more than just gift shops.