Hello!
We totally understand what you are saying, and indeed, for this quest the chest might seem expensive.
The valentine's day quest is usually fairly easy, and we try to make it complete without buying anything.
The chest is mostly for people who have access to paid Jewels and didn't have time to play, as a comfort buy. Also, when you buy jewels you can usually have about a hundred of them with one buy, so 8 Jewels is not so expensive if it can save you time.
The price is the same for all chests, that's why we kept it at 8 Jewels for the hearts, even if for this quest it's easier to stick with the banners