Pact status has a huge impact. In a four player Pbem, I was Morgan, opacted with the other three. Cost to corner was a little in excess of 8,000 ec Two turns later, I was at V with two of the other players, still pacted with the third, and the cost had skyrocketted to 60,000 +
I THINK your ECONOMY rating plays a role. I THINK also that the number of commerce techs you have plays a role. Lastly, I THINK that the commerce tech differential between you and the others plays a role.
According to my thinking
cost decreases as each of the above increases.