You can't buy an AOC. You can buy the shares in a company to which CASA has issued an AOC.
The company is worth the surplus of assets as per the balance sheet: Equity + retained earnings + assets - liabilities = surplus of assets.
Caveat emptor!
An AOC may have a good will value if it comes with approved personnel, Chief Pilot etc, and if the AOC authorises use of the aircraft, aircraft types and types of operation you intend to operate. It may then be worth what you are prepared to pay, which is not necessarily what the AOC holder asks.
Keep in mind that if you acquire an AOC as an asset, it will become an instant liability if CASA decide to send you one of their favourite Friday afternoon Show Cause faxes.
A basic AOC with no approved personnel and not authorising operation of the aircraft, or types of operation you intend to operate, is a piece of A4 paper only, which at law remains the property of the Commonwealth.
Last I looked, A4 paper was around $7.00 for a 500 sheet ream at Woolworths or the Newsagents.