K&F Concept via Amazon has the K&F Concept 77mm Variable Neutral Density ND8-ND2000 ND Filter with Multi-Resistant Coating, Waterproof in a lightning deal for only $46.74. This deal only lasts another four hours this morning!
This is sold by K&F Concept and fulfilled by Amazon. Other sizes are also discounted for this ultra-slim filter with a 7.5mm frame, adjustable from 3 to 11 stops.