I've done this before on Trinitycore 3.3.5a, I don't know if you're using Trinitycore 3.3.5a or not, so I'll just explain what I did.
I edited StableSlotPrices.dbc, and I figured out that editing the client side .DBC is not enough, you need to update your server-side .DBCs as well. Or else if you just edit the client side, it will only visually change the price but not actually change it server-side. So remember to update your server-side .DBCs