Doing DS 10H during Cata dropped like 100 or more gold per boss. 10 vs 25 shouldnt matter anymore because its per person but that amount is probably right.
25 man HC indeed drops far more gold than 10 man HC.
Whole Cataclysm is around 125 gold per boss per person. Doing all 28 bosses on 25HC will get you little over 3.000 gold.
Sizable but for time needed to kill 28 heroic bosses (some of them still pose challenge to inexperienced groups) is okay but nothing special either.