Did a small perl simulation of this:
Over 10000 successes, min money was £670, max was £3460, mean was £1,409.78, median was £1,350 and stddev was £336.62
Over 100000 successes, min money was £650, max was £4490, mean was £1,404.93, median was £1,350 and stddev was £335.05
So the answer seems pretty stable, I would expect to pay the median of £1,350, although as the large max shows, you can pay a lot more if you are unlucky with getting the last card.
Of course the vastly most efficient way is to collaborate with other collectors by swapping your duplicates for ones you do not have.
And, as a possible downside, often with these things there are not equal probabilities of getting each card.