Hi my name is ***** ***** Andres is offline currently.
If you would like I can assist you with your query, however first can you please clarify that you are looking to get a cost for a replacement key for your 2003 ES39, including on the coding so it will work on your car?
A new key with coding for a BMW 520i (E39 generation) should be around £150 from a locksmith\dealer. You can find a list of locksmiths in your area by typing your post code into the website here.
However my suggestion is you try and do it online, you can order a new remote key which is cut from the key code or photo you send them, an ebay seller is doing this for £50, for example the one I found for your vehicle is here.
Once the key arrives you just need to code it, which can be done yourself by following the you tube video here, or failing that taking to a locksmith who will probably code it for you for less than £60.