Custom metadata for personal usage - like when album is paid and how much it cost etc

Is there any way to add custom information about albums for my own usage?

Mostly I am keen about how much I have paid to some album and when I paid it. At this moment I just made playlist “2017-04” where is albums I have purchased this month, but the price is harder thing. Of course I can use tags, “Paid: 5 eur” or similar, but is there any better way?