Missing money

If the account's balance has gone, and if you hadn't asked iTunes Support to remove it, look at the account's purchase history to see what you spent it on : See your purchase history in the App Store or iTunes Store - Apple Support

You are sure that the $30 was correct and up-to-date ? Have you looked at the account's purchase history ? The only ways for a balance to have gone from an account are to have spent it, or if you had contacted iTunes Support and asked them to remove it.

The money was on another Apple ID than this. I could login to that account and reply to this message the username is cbjorkebo. Keep in mind it was from a gift card and I had used some money so it was roughly 30.

Was the balance up-to-date two days ago ? I've bought something on a device or my computer, on a different device my balance doesn't always reflect that purchase.

The only ways for a balance to have gone from an account are to have spent it, or if you had contacted iTunes Support and asked them to remove it.

Gift cards don't expire, and the balance on an iTunes account also doesn't expire. The account's purchase history will show what has been bought and how items were paid for ('store credit' or credit card). If you can't work out what you spent the account's balance on you could try asking iTunes Support and see if they can/will tell you : http://www.apple.com/support/itunes/ww (click on your country's flag, the country where you live, and which is therefore the country on the account ; click on Contact Support towards the top right ; click on Get Help under 'Talk To Us’)

This is the other account.