I was quoted £70+ by Pete @ Auto Nation for OE lower arms (inc ball-joints). He's normally much cheaper than your local dealer.
However, if you decide to just replace the ball-joints - Motaquip (owned by PSA, IIRC) ones are available on eBay at a very reasonable price. I've also been told that they're of OE quality. Can't comment from experience though so may be inaccurate.
When I did mine, I replaced the top mount bearings too (ECP) and it made the car feel MUCH tighter and more direct.
If you do replace the ball-joints, this may not be the best way to do it, but I followed this procedure:
- Wheel off
- Loosen strut top mount
- Remove 2 strut bolts
- Remove top mount and strut (careful of the CV joint boot and not over-extending the CV joint/ driveshaft)
- Dismantled the top mount and replaced the top mount bearing
- Removed the caliper and carrier (access to the ball-joint cotter pin through the hub is made easier/ possible)
- Removed the cotter pin
- Put a piece of wood between the driveshaft and wishbone (splitting the ball-joint is pointless unless there is an obstacle to stop the wishbone forcing the ball joint back together)
- Undo the 2 bolts securing the ball-joint to the wishbone
- Remove ball-joint
- Replace and reverse above procedure.
FWIW, I used a fork-type splitter.
HTH.