I once heard of a trick where you can get a variable 10-stop filter by using two polarizers.
Set 'em up in parallel and lose around 4 stops, turn 'em crosswise and lose 10+...
I haven't had an opportunity to try this yet--has anyone tried this? Might be both cheaper and more versatile than a 10-stop filter, especially if, like me, you need a large diameter one.
Just a thought,
-Brad