Ok, I did a little test with approximate values...
I've tested a small harvester (250 cargo) vs regular harvester (1500 cargo).
The small harvester traded with a player 7 sectors away. This player at the time had 47k ore. The harvester collected 38 ore.
So here is the equation I came up with: 47/(38/7)=8.7

Regular harvester traded with a player 14 sectors away. This player had 175k ore at the time. The harvester collected 277 ore.
Same equation: 175/(277/14)=8.8

Is this a coincidence? Maybe.
Is it worth testing to see if this factor is the secret key to trade? Absolutely!

How can you help us test it:
Step 1: find a player you wish to trade with and write down the amount of ore he has.
Step 2: count the sector distance (remember that AI ships pick the most direct root and they can travel in diagonal).
Step 3: under your details in diplomacy menu, look up your current ore values.
Step 4: after 1 round of trade is completed, go back to details menu in the diplomacy tab and write down new values of the traded ore.
Step 5: post back on this tread and make sure to mention which ship you used.

How to estimate:
(current ore on the planet)*(distance)/8.8=value