You ain't getting ripped off, those prices are spot on/cheap.
You CAN replace 1 shock at a time. I'm having to replace 1 strut only after my crash. It is not neccessary to replace both shocks, but bare in mind my Tein Flex shocks have only had about 10,000 miles on them.
1 shock can fail, maybe one side of the car took more punishment. Typically it is my left ball joint that goes with all the stanks and broken tarmac on the left side of the road.
If you are wanting to change to Altezza suspension, you would be best advised to speak to someone like Gareth at Envy or Dave at Pro-Lex UK. They would advise you as to what is best, but I would advise you to get Tein CS for ride comfort or Tein Flex for good handling but still OK for the road. Price about £900 for 4 shocks. You can get an electronic damping controller for about £250 also. Prolex or Envy can order them for you.