Cards values in personal collection

I am an Everton card collector and so many of our cards do not have a sale history, which leaves me a collection with an incorrect overall value. I've been racking my brain how to solve this issue and I think I have a good idea. Many of the cards with no sale history, or just one sale, is from the card I bought.

One of the reasons I use this site is to keep a value of my entire collection

Would we be able to have a feature that you could use the 'cost' you paid for the card as the value of it in your collection until such time it has a sales history? This could just be a check box when uploading the card?