Hello!
Arthurdubya, dream murderer here.
Gilding the sides costs a fairly big chunk of money, and is most cheaply done with a printer overseas in asia for obvious reasons. Depending on who's doing the job, it could take a couple of dollars per deck to 10 or 15. Quantity also matters, as a per-deck cost would be much higher for a run of 100 decks vs a run of 10,000. If you're printing decks overseas at say $5-7 per deck, an extra $2 per deck for gilding would be an extra 30% cost-adder.
A good chunk of money, but definitely worth it if you're planning on selling your deck at a higher price, or attracting a pricier market segment.
It CAN be hand applied to each card, but the process for 78 cards will take a devastatingly long time, and there would be little reason to do so. This would be a similar reason why doing different colors for different suits would cost more money. The process of edge gilding a deck is applied per-batch, so every group or color of cards would incur a separate charge.
Here's a video!
https://www.youtube.com/watch?v=_vvHph0FrUY