These are as close as I can remember.

Headgasket £30
Headbolts £20
Valve stem oil seals £14

Don't know about the tappets as I've never brought a set outside of a full cam kit

You will have lost a bit of water but it doesn't take much water to make the oil go mayonaise.

This is the header tank on my A4 when the HG went along with the oil cooler.