6 star max gravels = lvl 110
7 star max gravels = lvl 150
here is a picture of thor with max gravels at lvl 150, odin has gravels right now in lvl 140 so you can see it costs 100 gravel per upgrade when its higher
as for Poseidon, that's just to show it off to the Poseidon lovers
max gravels at lvl 100 and max astro at T10, something to look forward to when you are upgrading, the god of all morphs.