Converting TTRPG Currencies: A Nerdy Math Problem

I thought some people might enjoy this little math story, particularly because of the funny ending. I’m running old Basic D&D adventures (B2 and B5) using Mongoose Legend rules. Now Legend is one of the d100 family of games and they collectively have always been known for having a more realistic economy than D&D. Prices in D&D are based of the utility of the item to adventurers and not how easy the object is to make or how hard the materials to make are to get. Runequest and its descendants have always tried to consider these things when setting in-game prices.

So, the problem is, “How should I change the treasure awarded in the old modules to match the Legend economics?”

I’m a very analytic person, so I jumped right in. First, let’s look at the currencies. Basic D&D uses the Gold Piece as the standard, with 10 CP equal to 1 SP and 10 SP equal to 1 GP. While Legend uses the SP as the standard with 10 CP equal to 1 SP and 20 SP equal to 1 GP. To relate these currencies, I build a table of common items that have prices in both games, and I get this:

Basic D&D price, followed by Legend.

• Crossbow , 30, 350
• Longbow, 40, 200
• Short Sword, 7, 100
• Backpack, 5, 5
• Sack, Small, 1, 0.2
• Lantern, 10, 10
• Chain Mail Armor, 40, 400
• Plate Mail, 60, 1200
• Thieves Tools, 25, 75
• Dagger, 3, 30
• Rope, 5, 10

Famously, Runequest charges a lot more for armor and metal weapons. That was the stuff of the wealthy elites, while other items have the same or similar cost—just in SP instead of GP.

Making a scatterplot of this data, and fitting a linear regression line (forced through 0,0), I get:

My R^2 is not bad. The linear model explains 78% of the overall variation. That seems good. All I need to do to convert treasure form the old modules to my game is multiple the number of GPs by 12.8 and convert them to silver pieces.

But wait. I want the rewards of the adventures to be richer than what the old Basic rules would give. I want the place of adventure to be so rewarding that adventures will come, not just to make a living, but to become wealthy. How could I do this? I could give out two thirds more monetary treasure than the old module states.

If I do increase the rewards, then all I have to do is keep the treasure exactly how it is listed. The low value coins will be a little off, but they are just filler. When the monsters have gold, just give it as gold.

(Artwork copyright Daniel F. Walthall, used with permission.)