After calling a USA vendor, price was a major concern.
You have made your first mistake.
There are USA manufacturers, and I think they stay in business from aerospace and medical (I.E. nepotism and corruption.) Even these guys, I wonder if their value is more in the design side and for the assembly/inspection/testing and overall quality control part of the actual manufacturing process, things like expertise in setting up lines for odd ducks like assembling flex-rigid composite boards and x-ray inspection and stuff like that.
There are USA based companies who make your board, who have "affiliations with" overseas manufacturers. I.e., they take your Gerber and do the insanely difficult job of ordering the board (joke). And they can provide some real value, maybe, by assembling your boards for you.
In my search, I have found no legit manufacturer of flex in the USA that I would consider using unless I was spending someone else's money and a company rep was taking me out to strip clubs.
Unless the assembly and inspection and testing is something you can't quite figure out how to relay without meeting and going over in person, you probably don't have anything to gain from trying to find a US based company.
The biggest difference between flex and rigid is that small prototyping quantities are much more expensive. So you will spend a few bucks figuring out the physical properties and what you can and cannot do.
Also, I have heard first-hand from people who have designed boards in a way to nest together in order to save space/money. This might be a viable loop-hole for a given quantity/company. But they do not cram the panel with boards, like they do with rigid. If you submit a panel the same way you do rigid, it will be rejected. In short, don't spend much time trying to optimize space usage until you get a feel for what is acceptable. At first, I would focus more on getting the board into the smallest rectangle as possible. Perhaps, more detailed way to say it, if you can get maybe 2 boards to nestle together into a smaller rectangle, that might work. Then they will put a huge space between these. But if you design a board that has to nestle together across the whole panel, you wasted a lot of time.