Upkeep being "totally" random is much better. If it was 4% of the 7% of the buying price, it would be a far too low upkeep.
Gear worth 13,513 gold has an upkeep of 944. 4% of that is 37,76 gold. If a round takes 6 minutes, that's 226,56 gold to pay. Income with x1 and 6 minutes is 6x50 gold = 300 gold. Income of 73,44 gold.
Gear worth 53,507 gold has an upkeep of 3,743. 4% of that is 149,72 gold. If a round takes 6 minutes, that's 898,32 gold to pay. Income with x1 and 6 minutes is 6x50 gold = 300 gold.
Income of - 598,32 gold.
That's nothing. Alone for the Hourglass gauntlets in that gear set up (second case) the upkeep is 520 gold.
Imo any gold income is only meant for upkeep. If you use medium gear it covers the upkeep + often gives some extra gold /sometimes you lose some gold at a bad day. If your gear is more expensive you obviously lose some gold and may need some gold backup. If its much cheaper you will win a lot of gold, so thats the times you can save gold for times when you use more expensive gear.
I currently always know what I am about to pay after each round. I know the max so I usually guess I got to pay about the half (+/- 10%). And with medium gear you can be sure you're usually covering the upkeep while having x3.