Importing contributions always imports $0 in Amount
Check how you are importing your data. It is often worth doing an export to see how they data is held as this often provides a clue as to how the data needs to be structured to import. 

For example you may be importing as 5 when it expects $5 or vice versa.

Also be aware that there is a very helpful extension that extends what was possible with GUI imports. github.com/eileenmcnaughton/nz.co.fuzion.csvimport