TL;DR : The cost of one Quest Potion as of writing is 750 DEC (~5 USD) but if you buy using credit, one Quest Potion will only cost 0.75 USD
Before dwelling on the main topic, let us introduce the use of Quest Potion:
Quest Potion will let you gain five (5) additional Loot Chests for completing your daily quests.
The current price of one (1) quest potion is 750 DEC (~5 USD). However, if you will look closely, the suggested price is only 0.75 USD. This just means that the current price is not updated hence, it is advisable to use "Pay with Credits" than paying with DEC.
Yep, because DEC are pegged to a price of $0.001
That peg is maintained through the store always treating them as that price, and through the #play2earn rewards system increasing the rewards being distributed the higher the price of DEC is.
This is the first time the price of DEC has spiked like this, because of the airdrop of course, so things are a little wonky right now.