How difficult are these to do? Are they expensive? How do I search a topic??
Done mine today, took about 2 hours and cost £63 with TC (saves about £10).. for the £63 you get the four Cam Cover Gaskets, 16 "O" rings and a tube of black sealer 
Probably did .. but you've done it before I guess !!
When I did mine took nearly a full day, as I carefully read each printed guide and did things very slowly. I had problems with the cable tray and cut a corner .. bad news .. that side didn't go on properly and leaked like a sieve. In here that night asking questions .. got good advice ... redid the job the next day in half the time.
So if you know what you are doing ( I don't half the time!!) it probably is a couple of hours .. if you are like me .. don't rush and allow plenty of time. It is NOT difficult as I found ... I made my own problems !!!
I'll be doing the new beast as soon as the weather improves, and I'm far more confident now.