They do. Quoting from https://forum.grin.mw/t/mimblewimble-coinswap-proposal :
The contributed fees should then match the coinswap relay fee plus n mixfees. In Grin this gives the identity (with FB being the fee base of half a millicent):
|Out| * fee = (|Out| + n) * (1 + 21) * FB + n * 3 * FB + n * mixfee, or
mixfee = |Out| * (fee - 22 * FB) / n - 25 * FB.
For simplicity, we could require a fee of (22 + n) * FB, resulting in a mixfee of (|Out| - 25) * FB. In that case, for a mixnode to earn 10 grin-cents a day requires 225 daily self spends.