Filters that I use range from 1 to 16 stops of light reduction across the various formats I use ie: 39,52,77mm round and 100mm Lee. Normally I carry at all times at least a 2 stop filter and in summer here in Australia a 4 stop filter.
I can't give you an average price as some of the glass filters are custom made and would throw out the price. Most of my off the shelf ND's though cost around 50~100 USD per unit.