two different brickies might give you two completely different prices, depending on whether or not they needed the job, how busy they are etc.
as for the height i would say that you choose a height that you want and then the wall is constructed in size/width to suit that , either by rule of thumb or getting design help from an engineer or architect or asking the brickie.
would,nt your local building regs/planners expect some sort of design with calculations?