Take your car somewhere else for a second opinion, those style of inverted struts all have play in them and even knock when there is nothing really wrong with them.
If you look at your shock, the big chunky chrome shaft is not actually a shaft but the housing of the insert, the shaft is only about 20mm in diameter and sticks through the bottom of the strut housing and is held inplace by a nut. So the end of the shaft is held in place and the body of the inset actually moves. There are 2 slides inside the housing of the strut that are loaded up with grease, these slides get some wear and allow the strut to have some play
You can't get them rebuilt, well at least you couldn't when I was looking into it, because the shaft has a strange diameter size and they cant get new slides for the inside of the housing.
Yes that retail price is correct for factory STi ones, they're made by KYB and all tied up under contract so only subaru can sell them.
One of my good friends bought a second hand set out of a V8 Sti that had only done 50,000kms and they knocked too.
You can simply pull them off the car and undo the nut underneath the strut and slide the insert out them regrease the inside of the housing (with special grease), this helps the problem and sometimes the knocking goes away.
The knocking can be a little annoying but your struts are probably fine as long as the inserts are not leaking.
Talk to someone in the workshop at Autolign in Auckland (they do all the rebuilds on bilsteins and are them only authorised rebuilders in NZ)