The only real fees in Thankful are the transaction fees on the Ethereum network. These are at the time of writing $0.021 per transaction if you can wait up to 30min for your transaction to confirm (called 'safe low'). So if you're donating $10 across 10 creators, you'll be paying ~$0.2, or 2%, in transaction fees. If you're donating $20 across 10 creators, this becomes 1%.
For comparison, on Patreon donations smaller than $3 incur a fee of 5% plus $0.10, for a $1 donation this amounts to $0.15 (or 15%). Donations larger than $3 incur a fee of 2.9% plus $0.30, for a $5 donation this amounts to $0.45 (or ~10%).
You can see current transaction fees on ETH Gas Station.
Optional Thankful support fee
By default, 5% of the total amount you donate each month will be allocated to the Thankful Team so we can continue development. This can be set to less, or even zero if you'd like. It's completely voluntary and transparent in the donation process.
Further decreasing fees
We're looking into lowering fees even further by batching transfers into a single transaction. This would decrease fees by ~35% if sending 8 transfers in a batch.