If I plan on running my OS from it, it needs to be at least 32GB, and cost no more than $80. For 64GB I'm willing to pay $140. And for 128GB I'm willing to pay $256. 256GB should be priced no higher than $499 streetprice. I think these are more fair numbers to make a switch, only if following requirements are met:
- The IO ops are (much) faster than a regular HD, meaning it needs to random read & write small files faster than on a HD;
- The power consumption must be lower than a regular laptop HD.
Those are the only requirements to live with a tradeoff of higher price, lower diskspace.