Well... That's an intresting question. Please shoot me a PM if you would like some information, I have previously worked for GameSalute (a boardgame publisher) and I have participated in a dozen or so kickstarters.
In my eyes your fighting against an unknown demand- diffrent publishers have fought against this in diffrent ways over the last few years. You have a few ways to combat this, my recommendation is if you feel only a very small number of decks are worth manufacturing (200 or so) would be to get into contact with a company like The Game Crafter or Drivethrough RPG . They will allow for a smaller manufacturing run and after restoring your own stock levels you can leave the item as a direct item sold on their websites. Absolving your responsibility for manufacturing and distribution.
I believe the right price for these is about 15$USD, 10BP... But I would be willing to pay a little more if the presentation was very nice. I would follow the examples on this website and move over to a simpler "minimalist" graphic design that requires less color & retains character for upcoming osprey designs and to keep fulfillment afordable.
Add on sales like acrilic tokens and decks help my immersion and enjoyment of games greatly, I encourage you to outsource these or encourage your players to help with these offerings. (Etsy and the like)
Lastly, there are a number of manufacturers I have had some great experiences with including Panda and Whatz Games- Panda in piticular has been very helpful with a number of card expansions sold in booster format. It depends on fulfillment numbers.
Frostgrave has stabilized at this point, it may be a good time to introduce a spell deck accessory at this time.