These seem to have been discounted in different ways at different times, so I thought I'd collect them here:
Books 1-3 are £6.50 (in total).
Books 4-6 are £6.70
Books 7-9 are £6.50
Book 10 is 99p
Book 11 is £1.71
Book 12 is £2.56
Book 13 is £2.56
Book 14 is £3.43
Book 15 is £3.43
Book 16 is 85p
Book 17 is £4.19
Book 18 is £4.19
Book 19 is 85p
There are also a number of short stories at 49p each, which are included in the first three collections.
You can also get
all four Liam Campbell books for £6.00 and
all three Star Svensdotter books for £2.99 [Edit: Which also include the related short stories]