Bigga Is Betta interrupts the deployment as a whole, creating a lasting effect of a cost reduction of 2 plus dealing it 1 damage.
Nullify then interrupts the playing of Bigga Is Betta, cancelling this lasting effect. Nullify does not cancel the playing of Bigga Is Betta, it just cancels its effect. So the resource cost of Bigga Is Betta is still paid (though it be 0 in this case), any ability cost it had would still be resolved (none in this case), and BIB is discarded.
Once these interrupts to deployment have resolved, the original deployment then continues. Once you get to step 3 pf the deployment process (RRG pp 8-9), there are now no cost reductions to apply. At step 4:
- If you can't pay the deployment cost, the deployment attempt fails. You do not have to pay what resources you have - you either pay the full cost and deploy the unit or pay nothing and don't deploy the unit. You are also not obliged to use other cost reducers if you have them.
- If you can still pay the deployment cost with the resources you have, the deployment must continue. The process has started after all. But you gain the benefit that you no longer need to deal the unit 1 damage (because Nullify also cancelled this aspect of the effect.).
The following tutorial provides more detail on how the ability/deployment initiation process works: