Coby's 5star(500millions) bounty is an exceptional case due to his "hero" fame paralleling Garp's a bit, most likely due to his actions in the Rocky Port incident Oda keeps referencing to(Law, Kyros, Blackbeard). It's to the point Blackbeard thinks his fame might be enough to get the government clout in his future plans
And since it's usually 1 star for a captain, and assuming it goes crescendo in terms of numbers(on average) from Captain > Commodore > Rear Admiral > Vice Admiral > Admiral > Fleet admiral, Coby's unusual bounty might be what an average vice-admiral get in the Cross Guild bounty scaling
With that said, one thing to note is that it was mentioned that Cross Guild gets support from multiple criminal groups in their action.
We saw one in Du Feld who was the major support in MADS, and we have yet to get the meaning behind the crown symbol, so it's not impossible that their financing capabilities might go beyond what a normal group can manage comparatively to the World Government and very likely even that their influence will only keep expanding