The TMOv2 is a good example to look at. Another may be the Clarabelle(
https://geekhack.org/index.php?topic=106900.0) which seems to have moved about 150 units. From what I understand the first 100 sold in NA sold quite quickly, but the last 50 sold in Europe not so much, possibly because people balked at the necessary markup required for VAT. That was priced at $380 USD, and although it had the additional functionality of a numpad, both it and the arrows are southpaw which I think can be fairly considered a niche interest. Plus it lacks some of the punctuation and navigation keys on offer here which I think positions this as a more "mainstream" 55% board in comparison to Clarabelle's "Southpaw Mini-1400" style. The Clarabelle is also weightless.
For what it's worth I would not balk at $350 for this board, and in fact happily paid the £360 for the Clarabelle. I have no idea how many others feel the same as me (literally DOZENS of us) but I personally have a lot of interest in 55%-like boards. That being said, I would of course be happier if it cost less, and perhaps a run closer to the Clarabelle's 150 units would make that possible but I would totally understand a designer wanting to limit a run to what they feel comfortable fulfilling.
The only thing that stops it from being a shoe-in for me is lack of ISO support as that's sort of a deal breaker for me under most circumstances. In any case I will be watching with interest. GLWIC.