Hoya ProND 1000
Cheap filters are notorious for significant reductions in image quality, but this particular Hoya bucks the trend. Available for under $100 depending on the thread size you need, the Hoya ProND 1000 easily fits into most budgets. While some color casting is almost expected because of the nature of neutral density filters, the Hoya ProND had minimal color hues, even compared to pricier models.
Despite being affordable, the Hoya doesn't skimp on image quality. But, some photographers have noted that after about a year of use, the filter coatings started to break down, becoming unusable. While it's unclear whether that's due to environment like sea spray, the Hoya also has a bit larger profile compared to other models. While the design and durability may not be quite as nice, if you need an affordable ND filter, the Hoya will get excellent images at a more easily attainable price.