[Meads]

Stepping up one version will cost you significantly less, than it would a first time buyer. You're stepping up two versions, hence you don't get any benefit.
Where do you expect the cut off to be? Should someone coming from PT5 still get a crossgrade price?
It's the same with most software, isn't it? You can upgrade from the last version to the latest for a cheaper price. Not from a 4 year old version.

Look at it this way - if you buy PT10, you will most probably be able to crossgrade to PT11 for less, than it will cost first timers.