[Poll] Re-rolling Rare/Mythics in packs

Had an idea where you could use some kind of resource, Runes or Crystals, say, to re-roll a Rare or Mythic you receive from a pack for the chance to receive a different one you may not have yet.

Obviously, pricing such a feature would be a challenge to get right, but for initial arguments sake, lets limit the feature as follows:

• Can ony purchase a re-roll ONCE per card.
• If purchased with Runes, would cost 500x the cost of the pack you purchased with crystals. (IE re-rolling a Rare from a 600 Crystal Big Box would cost 30,000 Runes)
• If purchased with Crystals, would cost half the cost of the the pack you purchased. (IE re-rolling a Rare from a 600 Crystal Big Box would cost 300 Crystals).
• If purchased with both Runes and Crystals, would be half of each cost listed above. (IE re-rolling a Rare from a 600 Crystal Big Box would cost 15,000 Runes and 150 Crystals).
• Lastly, re-rolling Mythics would be double these prices.

These numbers are purely hypothetical starting points to start the discussion rolling. They should be non-trivial costs to encourage developer support, but also non-gouging to prevent consumer disgust and product abandonment.

Would this incentivise pack purchasing at the middle or lower pack sizes due to the cheaper re-roll cost? I can only see that as value for the developer, as bigger packs have higher chances of Rares and Mythics, but possibly higher rates of investment for re-rolls.