I'm not saying that you cannot photograph the Sun using the ND filter (do at your own risk). I'm saying that base on various resource I found, ND filter are not safe to be use for looking at the Sun using your eye.
Our eye are very precious especially to photographer, so be very careful and take all the necessary safety precaution... no harm to be a bit "kiasi"
By the way, I just check the price for the A4 size Thousand Oaks Solar Filter, it's S$29. Much cheaper than I thought.
Have a nice day.