First off, no one is going to pay 3000 gold for 3 ap and a bit of mana
We have runes for that.
Are you really this stupid?
The base item has 3 AP per level. Each individual rank of the core grants +20 AP, +1 AP/level, 150-200 Mana, and upgrades one of his spells.
The base item is up to 54 AP. After spending 3000 gold, it grants 168 AP, 500 Mana, and your entire kit is upgraded. The stats alone from the upgrades are worth nearly 3500 gold.
The only worthwile upgrade for viktor, is the E
DEATH RAY is your teamfighting bread and butter.
It might be a great first upgrade in most situations, but a movement speed steroid on his shield is actually really useful for kiting bruisers, and the ability to combine Veigar's E and Orianna's R CC is actually pretty powerful when you consider any amount of team AoE ever.
If Viktor would be reworked I would recommend
Q shield scales better with ap
W radius increase the more points you put into it
E the DoT was good, upgrade should be a range increase
R perfectly fine
Q shield is less important, it's a trade tool instead of making "tanky Viktor"
W radius is already freaking large. If it got any bigger, it would be inescapable.
E traded a guaranteed DoT for greater damage potential. Maybe if you use your W properly, you can hit both times like a good Viktor.