You know I honestly think some of these seed companies like the idea of their strains selling out. It keeps people wanting and wanting their strains, especially if they're great strains historically.
If demand is high, you can basically charge whatever you want, and people will buy (when they're back in stock).
Not sure how much mastery of simple economics goes into affixing price points or in the overall availability of product, but I think a lot of these companies sell-out, and stay sold out for a while, just to create an artificial (or additional) demand for their product.
Then, when the seeds are available, they're quickly bought up, in an attempt to 'get it while the gettin is good'.