At the point when you'd need to vertically scale up from shared spinning platters to a SSD, you probably want the performance boost of running on the actual machine. ServerBeach has some Power Servers on sale that you can upgrade to a SSD for like $160/mo, roughly the cost of a reserved Medium instance.
I suppose there's some risk one customer could burn out the SSD write-cycles then discard the node. Solution: charge for writes.