It won't add value of £500..but will cost you at least that.
Seat sell the double din centre console for £260 ish, you then need the sat nav unit itself (or aftermarket, still not cheap) and the fascia.
You then have the issue that it will remove the cigarette lighter and ash tray, unless you change the lower centre console too and that adds more cost.
I did the conversion as I found one at a good pice.
You can chop up a single din one and make it double din but that's a bit of a ****on tbh although it saves a lot of money.