You will pay the normal upgrade price in the shop.
The license of Cubase 7 includes all licenses before, so if you would like to run Cubase 4 as well you could do.
But that really doesn't make any sense.
MBP 15", i7 quad core 2,0 GHz, 16 GB RAM, OS X 10.7.5
Cubase 7.5.20 (64 bit), 7.0.7 (64 bit), 6.5.5 (64 bit)
Focusrite Saffire Pro 24 DSP (studio)
Focusrite Saffire 6 USB (live)
Core audio (mobile)