If the condenser currently fitted to your car is in such bad shape and you really want to have it swapped out, you must first ensure that the A/C system is evacuated of any remaining refrigerant. This must be done by a garage certified to handle/dispose of refrigerant. It really shouldn't cost any more than a tenner simply to have the system evacuated. Once this has been done, you could take the car to any competent garage to have a brand new condenser (with drier) fitted, then have the system refilled at a qualified A/C competent garage.
To give you an idea about how much a brand new condenser will cost:
http://www.ebay.co.uk/itm/Fiat-500-...533316?hash=item2a550cdf04:g:iNAAAOxy0x1TRVcm
I don't know what the labour time is for swapping out a condenser on a 500, but it was 1.5 hours on my Saab. I provided a brand new condenser with drier. It then cost me £50 to have the A/C system recommissioned, U/V dye, oil, bacterial agent, refrigerant and fully leak tested. This can only be done by certified garages.
I suspect that you wouldn't get much change out of around £200 for the swap on a 500 and that is at the lower end of the estimate. I paid £250 all in to have my Saab done and that was using a genuine GM/Valeo condenser and drier which I bought off ebay from a GM dealer. I sourced my labour elsewhere! Don't forget, you could have the system evacuated, buy the condenser yourself, and get a mobile mechanic to fit a new condenser and then have the A/C refilled elsewhere.
I have no idea where you live, but to give you an idea of what prices a specialst A/C only place would do the job for, see the following link. If you look at the table of vehicles listed, although the 500 isn't there, I suspect you'd be looking at a minimum of £375, but as I say, you might find somewhere cheaper.
http://www.autokool.co.uk/fixed-price-servicing
Clearly, you need to do some research and get some quotes, don't just plumb for the first quote given. Use the goodgaragescheme website to find a reputable garage in your area that is certified to work on A/C systems.