As someone who framed four photos (from small to 3 feet wide) a couple years ago -- I wound up using cheap $10-30 Blick frames and cropping my photos to "standard sizes", and spending $200 locally on the one that simply couldn't be cropped to a standard size.

I would definitely have been interested in online custom sizes, but with cheap materials -- plastic instead of glass, black faux wood frame. Not just for quality, but also for weight -- a glass covering over a 3-foot photo is crazy heavy.

There's definitely a big market of people who are post-college and want proper frames for non-standard-sized content, but simply want them as cheap as possible.