The answer to this question depends on the type of donations that you accept. ACH processing costs a flat fee per transaction, usually less than $.30 (regardless of the donation amount).
Credit card processing fees can vary based on your processor's fee structure.
Some processors simply charge a flat fee. Or, your processor may use an interchange fee structure based on card brand (Visa, Mastercard, etc) as well as type (Gold, Platinum, Air Miles, etc).
There also might be different fees like set-up costs, monthly or yearly fees, or other charges. Make sure that you ask your payment processor what fees they charge before you start using their services.