I've found that a used 10-stage ND filter is sold in exactly the size you want at a price of 15% of a new filter, except on delivery it's 21% of the price of a new filter on delivery.
The catch? It has a scratch that starts roughly in the middle of the filter and ends near the edge. The length of the scratch is 1/3 of the filter diameter. The scratch thickness is very thin, so the area it covers is infinitesimal (so I do not think that would affect the 10-level rating of the ND filter at all). It is still visible immediately, if you see the filter, so a fair scratch.
Does such a scratch affect the picture quality in any way? I understand that you can scratch the front lens element badly and the image quality is acceptable in most cases. Does that also apply to ND filters?
I will not use the filter if the sun is included in the picture. In addition, I will use a lens hood with the filter. But I may use the filter at 1:22, and with such a small aperture, scratches may be visible that are invisible at about 1: 5.
At an affordable price, the inner voice in my head says "GET IT !!!" But is there anything I do not understand about the condition of the filter?