The お is an honorific that makes this word a bit more fancy. When you're talking about money you gotta be fancy, after all. So, you're basically saying "gold" in a nicer way. What was gold back in the day? It was money.
The reading for 金 is different from the one you learned with the kanji. Makes sense since this is a single kanji with hiragana attached. Here's a mnemonic to help you:
You've saved up a ton of money. With it you're going to buy something awesome. Then, suddenly someone hits you in the knees with a bat. Now you have to spend all your money buying a cane (かね). Sucks for you :(
This photo is more important than money.
If you're careless with money, it's going to bite you in the butt.
You should be aware that this machine takes your money and gives you nothing in return.