Amazon has two selling plans. The Professional selling plan costs $49.95 (excl. GST) per month plus a per-item referral fee that varies by category. The Individual selling plan costs $0.99 (excl. GST) per unit sold plus a per-item referral fee that varies by category. If you plan to sell fewer than 50 items a month, the Individual plan may be best for you.
In addition the cost of your selling plan and referral fees, other costs may apply, depending on how you ship orders, whether you decide to advertise, etc.