Hello
The answer to your question depends on some variables.
Does the £620 include the cost of the bushes and the VAT?
Replacing the bushes can be a time consuming job. If the price includes the bushes, say £150, that leaves £420 for labour. If you reckon about £75 an hour that's around 5.5 hours'ish. then there's the alignment check Depends whether you think that's a reasonable amount of time but seems fair if it's an all in price.
I did my own bushes and put in Walrods, cost me about £80, but it took me more than 5.5 hours. There again, I'm an amateur.
I didn't think complete new arms cost £1200 a pair, seems a bit dear.
Give the OPC a call and ask them the current price.
With complete wishbones it's a simple job to swap them over but they will need to be re-aligned which is an extra cost.
One tip I can offer, if you have the wishbones replaced, ask for the old ones back. Even worn out wishbone arms go for up to £150 on Ebay, at least that would help a little with the cost.
ATB
Steve.
Edit: Personally I would go with poly bushes every time. Cheaper to do and less danger of them having to be done again in a few years (or less) I'm really pleased with my Walrods, They made a big difference to the way the car drives.
Gert Carnewal will also sell you some exchange wishbones fitted with Walrod bushes for 225 euros exchange per pair. Check out his website.
http://www.carnewal.com/