Bandwidth and coverage is not dependent. You can use any bandwidth for any coverage area. But If you are covering more area and your bandwidth is less means then you cant accommodate more users. If it is rural area then you can use less bandwidth to cover more area.
same bandwidth and infrastructure is not dependent. If you are covering more area then your infrastructure will increase.If infrastructure is increasing means cost also will increase. If you are purchasing more bandwidth then your cost will increase.
Conclusion is If you want to cover more simultaneous users then you should go for more bandwidth.But increasing of bandwidth doesn't affect infrastructure and coverage.
For example Small cell with 20Mhz can accommodate more users but with less coverage and less infrastructure and cost. Same no users can be covered in larger area using MACRO cell(20 Mhz) but more infrastructure and cost .