I usually use a very useful piece of abandonware called RFSIM99 . I downloaded my copy from a community college engineering professor's web site in San Diego California. It runs fine under Wine and its very easy to use for all kinds of filters, and its free. It does the math and plots curves and can even figure out the worst case examples due to component variations, and work with S-parameters.
Once you get the hang of it you can bang a multi-pole filter out in a couple of minutes, solder it together, and poof, it works.
Total cost, almost nothing.