I believe that the new rewards cards are an amazing value when it comes to the regular foil. However the new rewards cards are overpriced at every rarity level at least based on the power points for your dollar.
This is based on the fact that the new gold rewards cards have four times less power than the Untaimed Dice Rewards cards had. For example new Common rewards cards have a burn value of only 125 vs the old Common gold cards had 500 points burn value. Please see the table below for comparison of the pricing at each rarity level.
The reason for this price dislocation might be the fact that the current circulation of the new rewards cards is still pretty low and market has not reached its future saturation levels yet. As you see in the image above the circulation is in the 3-4k range for the new rewards cards vs 6k+ for old rewards cards right now. But we do know that the new rewards cards will be 20 times more common. So I would wait before investing in new gold foil rewards cards.
I realize that you get DEC bonus for using gold foil cards in battles, but that doesn't seeem to warrant the current market premium.
The new rewards cards have great functional attributes, but you do not need gold foil for that - you can just max out the regular foil cards for your summoner levels right now and wait for gold foil prices to come down.