When SSDs are cheaper than HDs, that will be the time to buy.
My brain aches too much to do the maths now, but I suspect that there could well be a long-term power issue to consider with solid state meory as well as the fact that solid state is transient if you want the high speed of access.
I guess the power thing is getting more even when considered up against disc devices - the density of memory circuits on silicon gives off a lot of heat which means that it's consuming & expending power. Discs also give off heat as they need to spin the discs so that is a close one. Given the density of the data per sq cm that we're getting on discs now I'd estimate it's a close call if you measured current consumed per MB of data stored.
To get high speed with solid state memory you also need to use dynamic storage, ie capacitive, which tends to leak & thus need refreshing, again consuming power & giving a potential for loss of the data if the power fails. On discs you're switching magnetic polarity so the data tends to be held without a need for power to keep it there.
All of this could be moot anyway, as we get high speed data networks & the wider availability of hosted services it'll be easier to let someone else worry about the storage while you just connect from anywhere & access your data.
BTW - Can anyone explain why my brilliant idea to make discs square & store additional data on the corners never took off
