From what the garage quoted price and time wise I would not of thought that they were swapping out a lower wishbone with the amount of work in volved and prob just more like a bottom ball joint!
The bottom ball joint is integral with the track control arm. Replacing individual bushes would require heavy duty press tooling which is beyond the scope of most garages, so the arm is generally replaced as a complete unit.
There are some very cheap pattern parts out there (the aftermarket sell a
lot of 500 track control arms), which may account for the low price. But be aware that many of them are basically rubbish and this is one component that's worth replacing with a decent brand such as Lemforder. Failure is very common on the 500, in part because the OEM arms are not of the best quality. You won't get a decent quality arm for £50+VAT. It might get you through the next MOT, but the rattling will be back within a year or two and you'll have to do it all again.
Avoid replacing with Fiat OEM parts as, apart from not being of the best quality, they are also not cheap.
Ideally replace both sides at the same time. The other side may not last long after the first one has failed, and this will keep the labour cost down since some of the work (bumper removal & tracking) would otherwise have to be duplicated.
Keep a close eye on front tyre wear once you get the car back; if it hasn't been tracked properly after replacing the arm(s); they'll wear unevenly and you'll soon be down the price of a pair of tyres - plus you'll need the tracking done again.
This is one job where taking the cheapest possible quote may not necessarily be in your best interests, particularly if you're planning on keeping the car for awhile longer. A good mechanic who will do a proper job using decent parts is both worth his wages and worth paying for.