It is not supply and demand, it is quality of the seeds. If any of you have ever grown Nivana compared to THSeeds, you would see the difference. The expensive seeds are going to be more uniform, consistent, and usually better buds.
The expensive seeds are usually strains they have been working on for like 2 or 3 years. Selecting mothers/fathers, doing major seed runs to see which parents do best together, etc. The cheaper companies will just any ol' mother/father and sell their offspring even though it is not consisten and they have no idea how it will turn out.
Both times I have grown Nirvana strains, which I will never do again, I ended up with every single plant looking different and taking a different amount of time to flower. It sucks when you get 7 females but 6 of the 7 are crap and the one good one isn't even that good. Spend the extra money and get quality genetics.