From their launch in 2006 the prices for the Action Man 40th range stacked up pretty much as follows:
Composite set (boxed figure + outfit card) £35.00
Boxed figure on its own £20.00
Outfit card on its own £20.00
The Escape from Colditz boxed set and the horses were also sold for £35 each.
The only exception to that pricing structure, as I recall, was the carded Judo oufit, which was sold for £35 as a "special" limted edition during a Modeller's Loft open day in March of one year (I can't remember whether it was 2007 or 2008).
After their initial launch, the Loft also hiked the prices of a couple of sets that could be purchased singly (the Argyll outfit card and one of the sailor figures - Ark Royal, I think) to £25 each because they banked upon those being rather more popular as individual items than they actually turned out to be.
As you have rightly pointed out, Ebay prices for the boxed / carded sets are, in general, ridiculously high these days. However, if a seller is keen to make a sale, perhaps to move a collection on, some items - especially decarded / loose sets - can still be picked up for a reasonable amount.
I have found that it's just a case of keeping an eye open to try to spot which sets you want to buy when they come up - and also having a bit of luck into the bargain.
I hope that this helps. (If I have written anything here that is slightly wrong, I am certain that our friends here will be able to correct the mistake).