You choose whether to pay after drawing the card and seeing what it is. If you draw multiple cards, you see them all before deciding how many of the resulting triggers you'll pay for.

If Nadir Kraken leaves the battlefield after its ability has triggered, you can still pay and get a Tentacle, even though you won't put a +1/+1 counter on Nadir Kraken.

While resolving Nadir Kraken's ability, you can't pay multiple times to create multiple Tentacles or to give it multiple +1/+1 counters.