Depends a lot on construction and whether the foundations are reusable (i.e. concrete or stone), but wood frame demo (an assumption, here) typically run $5-10/SF range. Obviously, if this is an open or screened porch than less than if a fully walled-in porch.
Fully roofed porch typically in the $30-70/SF range - lower end if screened or open, higher end in high cost areas or if enclosed. Of course, if fully finished as a sunroom or Florida room with lights and HVAC and such, then commonly more in the $70-100/SF range and up depending on locale and fanciness.
