There have been clarifications that expansions will be fully priced due of huge amount of work needed for them, and that there could be at least two - walking and landings.
Wow! Well that really does set the scale/scope of them then! ie: To warrant £30 or so to land on a planet there would have to be a whole lot more to it than just the visuals. ie: All the suggested stuff (missions/animals/vistas) we've talked about in the past!