You don't get SimCash for max storage expansion. At least, I didn't.
Max territory expansion gets you 35 in SimCash (Growing City achievement), but I don't see an achievement for storage.
Max storage is its own reward.
Besides being able to manage inventory effectively, I also enjoy selling expansion items my Sims continue to give me. They get snatched up before I have time to exit the Trade Depot. Like tossing raw meat to a pack of jackals.
