An M.Arch largely is connected with construction rather than interiors. Interior designing is also offered in design programmes. However the income range will be more dependent on what you do and where you are placed. Also again this will depend upon the firm - a well known one pays well whereas a smaller one wont pay as much. So one cant speculate on this. Check the job portals please. You will get a clear picture of the income range, the type of firms that recruit and more. You have to do the research and see how you should proceed.