First do the same with single fares, only having day fares and longer on their websites. That is why it came as a shock that Heathrow to Slough on the 75/76 was £4.60, I only had £3.15 at the time (and when I did go on it eventually l went for the £6 day ticket).
I believe First's app will give you the price of a journey when you plan a journey. Also for places with flat fares (such as Bath) they now give the price of a single online although I appreciate that flat fares are still the exception rather than the norm.