If a software eq will work for you
http

/listoffreeware.com/best-free-equalizer-for-windows/
Don't expect to fix the limitations of cheap speakers with EQ. You will be able to make them a tad better. When you boost 3db you are asking for twice the power and can stress and even damage speakers.
If you can't use software then a used EQ might help but as you say would cost more than you are looking to spend. Save you money and get better speakers or add a subwoofer.