What is the conversion rate of real money to gold?

Solution 1:

Here's the list of all currently available purchasing options and their gold per US Dollar, gold per Euro and gold per British Pound rates, sorted in descending order by efficiency. If the rounded gold/$ rate matches, gold/€ is used next. All figures are rounded to full gold. Note that these rates can be interpreted as the amount of gold you save when you buy something with real money (see below for an example). They are not related to the gameplay or card value you end up with.

• Curse of Naxxramas 5 wings bundle
  140 g/$ 159 g/€ 200 g/£
• Curse of Naxxramas 2 wings bundle
  140 g/$ 156 g/€ 200 g/£
• Curse of Naxxramas 4 wings bundle
  140 g/$ 156 g/€ 200 g/£
• Curse of Naxxramas 3 wings bundle
  140 g/$ 150 g/€ 183 g/£
• Curse of Naxxramas single wing
  100 g/$ 117 g/€ 140 g/£
• 60 packs bundle
   86 g/$ 95 g/€ 125 g/£
• 40 packs bundle
   80 g/$ 89 g/€ 114 g/£
• Arena entry
   75 g/$ 84 g/€ 101 g/£
• 15 packs bundle
   75 g/$ 83 g/€ 107 g/£
• 7 packs bundle
   70 g/$ 78 g/€ 100 g/£
• 2 packs bundle
   67 g/$ 74 g/€ 101 g/£

A few intereting implications:

• Naxxramas bundles all offer roughly the same ratio in USD, but in EUR and GBP, the 3 wings bundle is significantly less efficient.
• In GBP, the 15 packs bundle is more efficient than Arena entries, in EUR, it's reversed. In USD, Arena is only marginally more efficient because you save 10 cents for 10 Arena entries compared to 1 cent for the bundle due to the x.99 pricing.
• In GBP, the 2 packs bundle is more efficient than the 7 packs bundle.
• Even without the bundle discount, a Naxxramas wing is still more efficient than packs or Arena entries.
• The ordering of the 2 wings and 4 wings bundle is due to the same effect happening with USD Arena vs. 15-packs.

Here's an example of how the calculation works, using USD:

Let's take the 40 packs bundle. If you had purchased those packs with gold, it would have cost you 4000 gold, which means the nearly 50 dollars you spent saved you the 4000 gold you would have had to give for the same 40 packs. In other words, each dollar saved you 80 gold.
Compare this to the Arena entry. If you had purchased it with gold, it would have cost you 150 gold. The nearly 2 dollars you spent saved you the 150 gold you would have had to give for the same Arena entry. Which means, each of those 2 dollars saved you 75 gold.
In the long run (assuming you spend enough for the single transactions to not matter any more), it's thus more gold efficient to buy 40 packs bundles than it is to buy Arena entries. Of course, that only applies if you're interested in buying both of these in the first place. Many people advise against buying packs in favor of Arena because the possible benefits are greater, but it takes much more time to get from Arena what you'd get from a 40 pack bundle, so it comes down to personal preference and how much time you want to invest into getting new cards.