As mentioned before it's going to be difficult to remove without snapping the clips probably. I fitted a new mirror recently and the casing should come off if you push it straight upwards - coz to fit it, you push down and it snaps ito place.
I paid £18.36 for the cover from VAG - looks like part number 3B085/538BGRU. (that was for the drivers side) It came in grey primer and I got it sprayed at a local bodyshop for £40.00 inc VAT. Could be easier just to buy new covers, as I guess you would need to rub down your old ones anyway