I think the operating companies only release a limited number of lower-priced tickets and, in the 5 minutes you took to make up your mind, somebody else bought the last £36 ticket. If you can hold on for a while, it might be that, if the £57 tickets aren't selling well, the operator will offer further price reductions. But there's no guarantee, sadly.
I've tried various sites for buying tickets and found them to be much of a muchness. Now, I buy them all on line from East Midlands Railway, my local operator, and load them on the EMR smartcard. Although it's EMR, you can use the system to purchase and use tickets to and from anywhere in the country.