It is easy to get carried away with number plates, I speak from experience! My two cost £250 each which I think is ok. £500 is a lot of money for a single plate unless you have enough money that it doesn't cause you too much grief or it means a lot too you.
There are a lot of good plates still available for £250 on the DVLA website, also have a look on ebay.
The thing to remember is that a Fiesta plate is only as long as you own a fiesta, it adds nothing to the sale value of the car. You can see a whole load of M3 ### plates on non BMW's and it just doesn't look right, they lose all meaning when they're not on the car.
All IMHO of course.