At the $5/watt panel price we have seen, there is no financial justification for buying solar except for some niche off-grid applications. (This does not include off-grid homes which usually use very wasteful battery storage, and can get power cheaper via other means that don't do their generation whether you are using the power or not.) So the only real reason to get it has been a desire to be greener. Thus limited sales. A drop in price would cause a radical increase in sales.

However, it's quite common in manufacturing to have a base manufacturing cost that is 1/3 or less of the retail cost because there are other costs.

Solar has a magic threshold, however, which is the price of grid power. Get solar below the grid price (and the grid price is quite high in some places, like California) and suddenly it's not an ecological gesture, it's a sound financial decision. So there should have been a lot of incentive to hit that magical price, as long as demand can be met. (If demand can't be met, you might as well charge what the market will bear for what you can produce.)