These are large bulbs and usually grown in limited numbers, which would mean higher costs for labour, fertilizer, transportation and fewer bulbs in the same field as compared to say mass produced tulips. Also marketing and sales would be a much higher cost per bulb. Still I don't think this is as much a growing question as it is one of market economics.
For the most part this seems a very specialized niche market, so I'd assume that the high prices for some cultivars comes from lack of competition. As long as there is demand for the bulbs, but the supply is limited or controlled by very few sources you would pay high prices.
Here in Europe the common pink Crinum x powellii can be found for less than 4 Euro and a quick search revealed that the prices for it is reasonable in the USA as well. Out of season, but I found it for a reasonable cost here: https://brentandbeckysbulbs.co...
. So not all Crinums are very expensive. In this case you would have more competition and more supply as compared to more unusual or newer cultivars.
There are in fact many specialist bulbs that are very expensive, even though many of them are easy to propagate. So this isn't an unusual situation in any way.