Just as said in the title, I've managed to change respec cost for players to 0 using the mangosd.conf file, but I can't find a similar option for unlearning pet skills. Another thing too is that you can select the 'untrain pet' option and have it take your gold even if your pet doesn't have any trained skills. I'm using v13 of brotalnia's Light's Hope repack.
EDIT: Found the source of my issue. Turns out pet re-training costs are hard-coded in the core, specifically in the resetTalentsCost() function of Pet.cpp. Going to make a change in PetHandler.cpp so it always sets the cost to 0 rather than calling the resetTalentsCost() function.
-- uint32 cost = pet->resetTalentsCost();
++ uint32 cost = 0;