more than 50...

Sorry to hear that. If its any consolation you only lost 4 BTC (which is what you burned then)
Proof of burn. 4 BTC was burned.
Valuation is really up to the one who lost. It's human nature to choose the higher or market value to claim as a loss. In that case, 5700 XCP does come out to 50+ btc. Hopefully as someone who burned 4 btc at that time, he was not risking a large portion of his funds and can purchase some XCP to re-establish a position.