Adrian,
Make sure you ask for an estimate.
If the camera is unmodded, consider sending it away O/S to
http://www.spencerscamera.com/ and having it modded. Spencers charge about $300 for a basic conversion. You'll have to factor in freight. The mod will include a new filter to replace the scratched one.
This will be more expensive than getting the filter replaced by Canon but they will probably charge a fair chunk of the mod cost.
Joe