Assuming you’re funding via a credit or debit card, Paypal charges the payer the standard 2.9% plus 30 cents and the payee gets the full amount. So if you send Tom $10 on Paypal every month, it will cost you $10.59 and he’ll get $10. If you have a Paypal balance already they don’t charge anything at all to move money around inside Paypal.
On Twitch, the streamer gets 1 cent per bit you cheer with, but bits cost you more than that, starting at 1.4 cents per bit and then dropping from there as you buy in bulk to a minimum of 1.23 cents per bit when you kick in $308.
Probably the best choice short of setting up your own payments with Stripe would be Square Cash, which charges zero fees for payments funded via debit card.