Currency exchange gold listing cost, based off quantity/type of currency taking, doesn't line up.

I understand there's a reason to have higher quantity trades cost more, but the current implementation doesn't make a lot of sense. In theory, trades going through should be relatively equal for both participants of the trade, so having someone being charged a lot more just because of the type of currency they are using doesn't make sense.

For example:

1.
I want to sell a power rune for 160c, this will cost me 4000 gold.
I want to sell 160c for a power rune, this will cost me 125 gold.

2.
I want to sell a power rune for 160c, this will cost me 4000 gold.
I want to sell a power rune for 1div (roughly same value), this will cost me 250 gold.

3.
I want to sell a power rune for 160c, this will cost me 4000 gold.
I want to sell a power rune for 1 mirror shard (roughly 20x value), this will cost me 1250 gold.

I don't think this is a major issue and I think the currency trading has been implemented very well, but it is still nonsensical. You could argue that in a real world application more quantity of stock might take more storage space in the trading process, but it doesn't make sense in the game. I think it is just implemented this was because it's very hard to predict conversion factors in an evolving market, much like the conversion factor on the trade site are also an estimate.

One suggestion I could make, that I think would be an improvement, is to have the listing price be based off the average of the buy and sell price. So in example 1. that I have given, the price of both listing would be closer to 2060 each rather than lopsided.