Alright, assuming we're on the "Medium" XP track...
That means that by level
8-9: 24,000xp / 12,000 Godrobe XP/GP.
9-10: 30,000xp / 15,000gp
10-11: 50,000xp / 25,000gp
11-12: 65,000xp / 32,500gp
Which means in the next 4 levels we'll get 84,000gp worth of Godrobe upgrades. These would be, in addition, I would imagine, to other equipment we obtain, of course...
I could go along with that if (and of course I'd love to hear what other people thing), say, when we actually level up, we could choose a single magical item we've obtained and, at some cost you determine, meld it's abilities into the Godrobe. I say this because, inevitably, as we adventure we're going to find or create magic items we end up really liking for one reason or another, and we'd really like to have them in the Godrobe instead of lugging them around.
I also bring this up because, I would imagine, that the Godrobe takes up most, if not all, "Slots" on our characters... I mean, by appearances just from the show, it summons a pair of boots and gloves and at times head-decorations etc... I would imagine a Godrobe would resist us wearing normal magic items. Allowing us to absorb items into the Godrobe as well as customize it with the XP-Gold thing, would make them customizable and scalable without losing out on the awesome magic items we find as we adventure.
Either that or some kind of 1 Godrobe XP/GP per 10gp we sacrifice during some carnal ritual or something. (aka, we sell a whole bunch of slaves and we make 400,000gp... split amongst the group of 4 of us, that would be 10,000 Godrobe XP/GP. Another idea. It would basicly mean that we'd be selling off most things we loot, all for the furthering of our Godrobes, since if we can't wear the magic gear we find for the most part, we'll be selling them (or somehow ritually sacrificing them for gold)... and we'll grow more and more obsessed with pillaging and looting for our Godrobes... just thoughts. Lol