as far as i have heard from driving instructors, the average would be about 24 lessons... which is what i ended up having, although im sure they do squeeze perhaps 1 or 2 extra out of you... it all depends on how confidant/aware/adaptive you are to things. no one driver is the same, but if i were you, id set aside enough to cover 24 lessons, aim for at most 20 (providing you are not driving outside of lessons with an instructor) and if at that or less lesosns you pass, then spend the left over on celebrating!
i paid £18 for my lessons, this was about 8 years ago now. prices at the moment seem to average about 20-25. there are special deals which can get you cheaper lessons... but most special deals will have some sort of requirements that end up in you staying with them and paying through the nose for the rest of the lessons, or paying up to again, average cost of every other instructor...
dont be tempted to get a cheap deal then change instructor then get another cheap deal then get another instructor...