@davidfor: An example webpage source would be
HERE. Most of the webpages look like this and it has the price information. So would this be enough as source? Use a page like this as default. But again the problem is the price may be different to what I actually paid so may not be viable anyway? Or would you need to put each webpages for each book? If this is so, then probably not worth doing it.
Regarding Automator, I think I can get it to find all the receipts and extract that information to CSV which could then be used with the Import List Plugin. I remember seeing someone doing something similar to this, so should be possible. I will need to play around a bit.
Inukami