How much to pay for a DeviantART high resolution file?

An artist on DeviantART that I think is really talented offered me to send me the high resolution file so that I can print it on a really big canvas (which DeviantART shop does not offer). He told me to give him whatever I thought was fair, but I have no idea! How much should I give him per files? (I'd like to purchase 3 of his photoshop drawings).