You have a point with makers concentrating more on larger knives, but that is mostly because the buyers do so. Also - larger knives are often used for wider field of task where the performance of the blade (or lack of it) shows more. If you take the custom knives on the cheaper (and simpler) side - you can get a 240mm gyuto for about $400. From the same maker a 150mm knife will probably still cost at least $200 and so many (of us) rather save up on that gyuto than get smaller knife made.
Still - I think that it would be tough to make custom knife in that price range and still make profit out of it - at least in the western world. While many custom makers charge by the blade length, there is certain amount of fix costs that do not scale with the size of the knife and so there is certain minimal price that is probably hard to go below. At least so I think.