I too have been priced for the rocker cover complete with gaskets by Vauxhall.
I did however do the job myself afteer a bodge job by a non Vauxhall garage who left the job to the junior mechanic who changed the n/s cover without properly removing the obstructions around it. In removing the cover he cracked it but replaced it filling the crack with sealant only for me to later find it still leaking. I decided to do the job myself where I found the crack... not a happy bunny was I. I then had to buy the whole cover at the price you stated.
At the same time I replaced the rocker gaskets on the other bank while all was dismantled. Not wanting to pay for a whole rocker case I purchased gaskets from motor factors. Before fitting them i carefully checked them against the VX gaskets on the new cover and they were exactly the same and good value for about £10 for the two it takes for each bank. I have fitted these and they have been fine since, remember to use the right gasket sealant, the black high temp sealant.
Also get new rubber rings for the rocker cover bolts, these I could only get from Vx and they were only 18p each, you need four per cover.