Dealing with Royal Mail can be a problem, especially if you are trying to make a claim or complaint. The value of the items is what would be considered fair, so I would suggest that you check out eBay, Google, Amazon and similar sites for a price comparison as to the worth of the items.
Take print-outs on anything that you find, because you might need it, as evidence for the claim.
Consumer Direct have a section offering advice about Royal Mail, but frankly I got very little help from them, when I wanted advice. I find that Royal Mail's own complaints procedure isn't all that brilliant in trying to resolve issues.