The conversion ratio is determined by a few things.
-Cost of amp dyes
-Cost ratio of enhancing to +10 using runes vs. using in game gold
-Cost ratio of enchanting <= +10 using runes vs. using in game gold
-Loosely tied to drop increase to +LUK items
While this may not seem always directly convertible, since we live in a large market with a lot of different things going on, it actually does. Since gold sellers sell most of the gold directly, and dye amps are often the "legit" conversion choice for players, usually their prices most closely reflect the conversion ratios.