Upcoming Medal Loan 1131577799

I'm not a medal dealer or a collector, but the valuation seems high.  GSM with NI clasp and SAM awarded to a Royal Marine went for £860 at auction last year - even allowing for the fact fewer RE than RM were awarded the SAM, making it a bit rarer, I don't see how the addition of a Military MBE and the ephemera makes this group worth £6,550, let alone £20K.

But then I don't understand the whole collecting thing  ...