10 stops of darkening is often too much for waterfalls, as was already explained here. I personally believe that they're better for blurring out motion in a seascape.
$60 for a 10-stop ND is cheap. There's no way around it - such a strong darkening effect isn't well suited for cheap filters. However, there's a gem from a company called Camdiox: http://camdiox.wix.com/camdiox#!product/prd12/2191246525/camdiox-cpro-smc-nano-nd1000-filter
They also have a variable ND, which may be more useful for you (though it is more expensive).
I'd get a filter for the lens with the biggest thread size you have, and just get cheap step-up rings for the rest. It's better to have it and not need it, than to need it but not have it. And I'd personally shoot a lot more landscapes with the 18-140 than I would with the 35.
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