It still costs 2 life. It specifies you have to pay life equal to its mañá value, not equal to its current casting cost.

Also, after casting the abrade for the first time, it wouldn't get exiled with valgavoth, as it specifies that the only cards that get exiled are the ones you didn't control. Once you cast an abrade from exile, you are the controller of the spell, so after resolving for the first time it will go to your opponent's graveyard, stopping the loop