The intended price for a quest potion is supposedly 1 dollar each. When buying thru credits, we are not actually saving, we are just paying the exact amount that we should pay for every quest potion. But why isn't it worth it to buy quest potion using DEC? Well, DEC's price was originally 1000 DEC for 1 dollar. But, because of the SPS airdrop being implemented for 1 whole year, and people get more airdrop rewards according to the amount of DEC they have in-game, people are hoarding DEC in-game. And this affects the DEC value by a ton.
Here's why... The price of DEC is dependent on the amount of DEC distributed to all players, and if more and more people are hoarding DEC instead burning them, the price of DEC will surely appreciate.

This conclude to the following:

1. The game will push you to your limit and to become more competitive.
2. The value of DEC will increase (maybe) because the minting of DEC and burning of DEC will  be equal now. ( Props to the developer for making this move as early as possible and its a smart move from developers to prevent multiple account abusing the quest potion to earn quick money)
3. Grind as much DEC as you can