B&H has a Tiffen 77mm variable neutral density filter for $99.95 (reg. $149.95) in limited supply with free shipping in the US.
Even if you a lens with a smaller filter ring, a 77mm ND filter is the way to go. Buy a step-up ring kit to make it work on your lenses. They’re cheap and everyone needs a step-up kit in the lens bag anyway. You can also get individual step-up rings from any lens size to step up to a 77mm.
Check out Tiffen ND filter deal here at B&H Photo.