I do multidrop for a parcel firm in the mornings. Afternoons (when needed) and weekends at the Flying Club Instructing.
A good month might make £500 from instructing. Typically £350 to £400. You will inevitably loose quite a few details to weather and a fair few no shows.
What you must try and do is make sure there is some form of no show/cancellation fee for students. If they don't turn up without giving reasonable notice (24 hours) then they get billed a cancellation. About £30 is the right amount - you get £20 the club gets the rest.