The costs are excessive atm, but as the technology reaches mainstream, the costs will plummet.
At this point, it's still only tech nerds which have SSD's, the average joe on the street is still comparing capacities, and don't understand what an SSD is anyway.
In 2 years, SSD's will have caught up to HDD's in capacity, far surpassed them in speed .. and people will be wondering why on earth a set of mechanical spinning platters makes sense.
At the capacities that HDD's (and soon SSDs) have reached, most people will be more than satisfied .... the vast majority of people wouldn't use the full capacity of a 128GB SSD .. when a 512GB SSD is in the affordable range (2years) ... it won't make sense to install >1TB HDD's for the vast majority of the population.
Additionally, with the advent of digital streaming, increasing network bandwidths, online storage .. there won't be the huge demand for local storage.
Maybe 2 years is optimistic for the complete oblivion of HDD's, but now we're in mid-swing of the SSD revolution, and in 2 years the average joe will really be evaluating whether they really need a big slow mechanical HDD or a smaller, but still large super-fast SSD for the same price.
I know that I've just been waiting for SSD's to mature before I have them in all our office machines ... if it's wasn't for the bad experiences with 3 of 3 SSD's (OCZ/G-Skill) dying within 3 months of installation .. everything here would be SSD ... except maybe the raided file server which keeps all the files.