It'd dependant on the Pokemon's base friendship, since wild Pokemon cannot gain happiness. Most Pokemon's base friendship is 70, which means Return's power would be 28; though some Pokemon have more or less. For example, most legendaries like Mewtwo (who has 0) have less, more cute or overly affectionate Pokemon like the Cleffa line (who have 140) have more. This has a list of Pokemon and their base friendship; to get the base power of Return, just divide the base friendship by 2.5.
As for the Mix Battles I am unsure, though since it is technically not trading the Pokemon's happiness wouldn't be reset, so the Pokemon's friendship (or happiness if you call it that) would be unaffected.