What's the price difference in producing different quality game cards?
There are numerous podcasts that cover this sort of thing. Multiple kickstarters have gone into detail on it. I think Dice Tower Productions recently talked about card stock pricng (Episode 450-453 somewhere around there). I think Two Rooms and a Boom's Kickstarter went into detail in card quality because it was certainly an issue in their highly anticipated yet highly delayed product.

Card Quality can vary from a number of ways from the finish which can be variations of smooth glossy or textured matte, the core (notably black core isn't see thru in white areas), the construction (paper or plastic) and the weight (thicker cards weigh more, gsm in the listings below means grams per square meter. Those are just some of the variations in card quality. 


  The black core means that no light can pass through the cards regardless of what is or is not printed on either side (TGC)


MakePlayingCardsNow.com's website gives some pricing information that can make a nice jump off point for estimation. 



50-99 Bridge Cards (Per Deck)

270gsm promotional quality          $6.55
300gsm premium quality              $6.85
330gsm superior smooth quality      $7.65
310gsm casino quality (French)      $7.45
13pt White Plastic (100%)           $12.85


50-99 Poker Cards (Per Deck)

270gsm promotional quality          $6.55
300gsm premium quality              $6.85
330gsm superior smooth quality      $7.65
310gsm casino quality (French)      $7.45
13pt White Plastic (100%)           $12.85


50-99 Lard Cards (Per Deck)


  Our large sized playing cards measures 3.5" x 5.75" and are popular for marketing needs or used as flash cards in school


300gsm premium quality              $9.80
330gsm superior smooth quality      $11.40
13pt White Plastic (100%)           $23.80


50-99 Tarot Cards (Per Deck)


  Tarot cards measures 2.75" x 4.75" and comes in deck sizes of 78. Perfect for those into tarot card reading or for promotional purposes.


300gsm premium quality              $11.20
330gsm superior smooth quality      $14.00
13pt White Plastic (100%)           $27.20


50-99 Mini Cards (Per Deck)


  Mini size playing cards measures 1.75" x 2.5" and are great as promotional client gifts or re-sell at retail. They make great gifts to friends too.


270gsm promotional quality          $4.20
300gsm premium quality              $4.50
330gsm superior smooth quality      $4.90
310gsm casino quality (French)      $4.90
13pt White Plastic (100%)           $8.50


50-99 Micro Cards (Per Deck)


  Micro custom blank cards measures 1.25" x 1.75" and comes in deck sizes from 54 to 360. These micro card decks are the smallest in our range of customizable cards. They are popular for small card figurines printing for board game or can be used for any other purposes.


270gsm promotional quality          $2.40
300gsm premium quality              $2.70
330gsm superior smooth quality      $2.90
13pt White Plastic (100%)           $4.70




There's even more sizes if you care to go to the website, and of course more pricing schedules. The numbers range from 1 to 15000