The amount of food, lumber and gold you can transfer to a teammate is the same until level 175 for some silly reason. The amount of gold you can send still goes up a tiny bit until lvl 186 until it stops, while food and lumber transfer rates keep increasing at the normal rate. Why? At higher levels this gets way out of whack.
This is my max transfer rate of each resource at level 420. It took me 2 and a half hours to transfer enough gold to my teammate for her to level her primarch. My suggestion is simply that the max gold transfer amount needs to increment at least at the same rate as food and lumber, if not more so given the costs of leveling primarchs.