I sorted a trip for my club with Poplar Diver this summer.
Most things have gone up in price in the last approx 20yrs, £350 to £550 is probably a fairly typical inflation lift since then, I am no economist though.
Agree St Kilda is the most stunning place I've dived in open water, it's pretty d*mned impressive above the water too.
We had a bit of an adventure getting there, force 8s obliged a fantastic circuit of the inner and outer Hebrides before we made a dash for it over an undulating magic carpet ride courtesy of the open Atlantic.
Coming back, via Barra, it was as calm as you like.
Cost, once you are on the boat, unless you are buying booze on the boat-----surely not--------there is no extra cost.
PS, the boat ran out of wine on our trip
If you drink in the Puffin, you will have to drink a heck of a lot before you notice your wallet getting any thinner.
Every UK diver should make the pilgrimage at least once before hanging up their flippers
Cheers, Malcolm.