Prime music unlimited is cheaper if you're already an Amazon Prime Member.
If you're already an Amazon Prime Member then it costs only $7.99 per month for a monthly subscription to Amazon Prime Music Unlimited or if you want to pay yearly it's $79.00 per year.
If you're gonna keep the Prime Music Unlimited Membership longer than 6 months then it's worth to pay the yearly cost which is cheaper and saves money.
If you're not an Amazon Prime Member the cost of Prime Music Unlimited is a bit higher in cost and costs $9.99 per month.
I have the yearly membership after I decided I loved it and it saves me money and I'll keep it that long and probably longer anyway.