Basements are expensive. They involve much excavation, soil disposal, waterproofing of walls and floor, possibly special drainage and in some cases automatic pumps.
However, if your site has a steep slope, a partial basement may be a good idea. Up to 3 storeys costs are much the same, but the higher you go the more structural engineering considerations can add to costs.
To approximate square feet as square metres divide the square feet by 11 (to be exact divide by 10.764)