Here is my 2 cents for what it's worth. I have 3 sets of ND filters. 2 sets were sent to me in exchange for an honest review on Amazon. And the one set I bought, the Polar Pro Cinema Series 6-pack (V.3)
The two other bargain bin sets had a problem with color cast, lens flare and vignetting. Both sets looked as though I was filming out of a shoebox. There is definitely a difference in the quality of HD glass, evenness of multi-coat, fit and being truly color neutral. The other 2 lesser filter sets claimed to be multi-coated but after the lens flares I experienced, I really doubt after knowing the extra expense of the MC process compared to their price point that they were actually even coated at all.
The PolarPro however are amazing, true colors, crisp and clear. Another plus with the PolarPros is their customer service. They are top notch and if any problem however slight occurs, they handle it immediately. A lifetime warranty from a company who is not only an approved vendor here on Mavic Pilots. But the owner and founder himself chimes in on posts here to answer questions/concerns directly.
When people talk budget ND filters I look at it like this. You bought a $1000 drone why now are you choosing filters based on lowest price and not highest quality? The one purchase that will make or break the most important part of your hobby, the image.
No one has ever regretted buying quality.