I want to book flights from UK to Spain for my son and have found that the outward journey is cheap but the return journey is much more expensive. Why? It used to be that the outward journey was more expensive but now it seems to be the other way round.
The price of a ticket mainly depends on the number of seats on each flight that have not yet been sold. If the return journey takes place in a time of peak demand while the outward one does not, the price difference will be as you have described.
Sunday evening flights are usually the most expensive ones, Friday evening somewhat less so, while something like Wednesday around noon will stay cheap for a long time. These patterns also tend to shift if there's for example a national holiday on one end of the trip, so a holiday on a Monday can erase most of the Sunday evening peak.
See this excellent answer for a really detailed explanation of flight ticket pricing.