I spent the last 90 min thinking of possible equations and came to the decision that an equation to find trade values without at least 100 outliers would be near impossible and not worth the time.
the DI/TEXP/FOOF/ROA throw off the equation for the rest of the cards, plus there are about 30 individual cards that will almost certainly be outliers as well.
What would really be nice is a spreadsheet containing 3LG, covenant games, and 2 of the active sellers' (on here) prices, plus the average of them all
Fixed set/same rarity cards do not throw of the equation for other cards, they throw it off for themselves. Each one has the same rarity, say, 5. This causes cards such as Grapes to be undervalued and those such Asa's good reign to be overvalued. However, that's where supply and demand comes into the equation. In addition to determing set based availability, one must look at how hard it would be to trade for said card instead of opening packs for said card. In this way, the equation should end up largely correcting itself.
I'd be interested to see what cards you think are an outlier. I currently only see Women's Errors and Faithful Servant as outliers. I don't think running 5 cards as outliers is that big of a deal.
