A query that returns a single row using an index performs one I/O for each index level plus one read for the data page. The optimizer estimates the total cost as one physical I/O and one logical I/O for each index page and the data page. The cost for a point query is:
Point query cost = (Number of index levels + data page) * 18 + (Number of index levels + data page) * 2
optdiag output displays the number of index levels.
The root page and intermediate pages of frequently used indexes are often found in cache. In that case, actual physical I/O is reduced by one or two reads.