Being so closely related to the Imperial-II class ships means they’re not likely that much more expensive. So scaling up from the Imperial-class to the Allegiance class, based on armor, guns, reactors, shielding, et cetera, I’d guess it’s in the 200,000,000 to 250,000,000 range.
By comparison, the Acclamator-class assault ships used by the Republic were 29,000,000 a piece but with fewer guns, worse armor, and significantly lower tonnage (they were less than half the length) they would be ruined by an Imperial-class ship. Cost of capitol ships is part of why the Rebel Alliance relied so heavily on smaller Star Fighters. Basically, a fully loaded Imperial-class Star Destroyer is not cheap. They’re crewed by nearly 37,000 imperials and they carry, among many other things, 72 TIEs, 30 AT-STs and 20 AT-ATs. The Empire eventually fielded more than 25,000 of these and someone in one of the EU comics (I can’t remember who) said that almost half of the Empire’s budget went into paying for the Star Destroyers. The Imperial-I class ships cost slightly more, right at the 150,000,000 mark. Imperial-II class Star Destroyers just under 150,000,000 credits.