15/15/70 Valve/Creator/Mod Creator

15% to Valve for merchant fees(this is already crazy high compared to the credit card processors and other payment processor's fees). They shouldn't get the full 30% they charge for game publishing since they already offer Steam Workshop for free and mods don't have the same visibility as games in the store.

15% to the game creator for on-going royalty fees for re-purposing of assets/technology. They would also receive final say in what mods are or are not allowed in Workshop, paid or free. They retain ownership of their original creation and any use or derivation of it.

70% to mod creator.

It is believed that Valve takes 30% from any commercial transaction (google it, although none of the link I found seemed 100% reliable).

IMO, Bethesda should not get any money (see my post)

That leaves 70% for the modder. Seems acceptable to me.