Actually, in terms of pure choice, we have...
280 choose 50 = ( 280! / ( 50! * ( 280 - 50)! ) ) =...
71,077,889,363,964,271,283,684,343,950,838,998,128,149,871,121,723,976,480 different possible maximum perk configurations.
:P
EDIT:
And to project the combo # for the alternate interpretation, assuming a raw level cap of say 80,
280 choose 80 = ( 280! / ( 80! * ( 280 - 80)! ) ) =
297,149,965,708,674,792,509,136,871,675,892,284,059,750,252,596,044,007,787,603,699,214,237,140 different possible maximum perk configurations.
This is an accurate calculation only if the 280 perks are completely independent. But it sounds like some of them are upgrades to perks. So, for instance, if perk #112 is an upgrade to perk #98, then it's not the case that you can combine #112 with 49 (or 79) other perks
not including #98. So the number of possible combinations is somewhat less (how much less is difficult to say, without knowing more about what the perks are).
Nevertheless, it's still almost certainly going to be an extremely high number.