There is a price threshold, at least for me. Right now HDD's cost say 5-6 cents per GB. Once Flash starts to hit the 3-4 cents per GB level (assuming HDD have gone below 1 cent/GB), it starts to make up for itself for many applications because of the performance/reliability factor. Right now I have say 200 'videos' per drive on a single 1.5TB drive (2TB just recently became more cost effective if you wait for the $99.99 sales at newegg). However, if that drive goes, it will tend to go in a catastrophic way, and I would lose all or a large portion of that drive. The hope/expectation is that a Flash drive sitting around without moving parts, would be much less likely to have a catastrophic failure like that. More likely for Flash is you would lose a single file or two. When you realize it might take two to three months to recover a completely lost HDD (redownload the 'videos'), at a reasonable download rate, it makes a lot more sense to make that switch as soon as you can.
A naive analysis of my Comcast Business Class service: $59.99 per month, bandwidth usage 1 TB, but lets say 1TB. A naive analysis already values a GB at 5-6 cents per GB just on ISP costs alone. You could say knock this down to 2-3 cents if you assume that even if you stopped downloading, you would need to spend $30 a month for decent internet if you didn't want good bandwidth.
Plus the labor involved in organizing your 'video' downloads, etc. might run into 4-10 hours per drive.
Once that threshold is met, and the perceived reliability of SSD versus extra cost becomes negligible in the overall cost, a lot of people will be jumping on SSDs, which should ultimately drive the costs even lower.