It really depends.
When I’m sending out Loot, it ranges from £15 for the lighter parcels, and stays there for a bit.
It definitely helps that GW doesn’t do metal, as of course it keeps the weight down. But, price is also dependant on parcel dimensions.
Heavier stuff can sometimes be cheaper by Courier.
Very broad, general rule of thumb? If I order from Element (my preferred online discounter, others are available), then the discount of 15% should, broadly, match the postage cost when sending it on.
Now it’s at this point I’d normally pimp the Loot group, but we stick strictly to Exclusives and limited run stuff, so this sort of stuff whilst understandable isn’t a service we offer there.