It depends on how your indexes are formed and on your database's optimizer, but something like this might work:
SELECT COUNT(*) FROM table WHERE numeric_column <
(SELECT numeric_column FROM table WHERE your_criteria)
In my version of mysql, subqueries aren't supported, so I actually have to save the subselect results into a variable, then do the second select, but EXPLAIN says this similar query is reasonably well-optimized:
mysql> explain select count(*) FROM faar_homes WHERE price < 80000 \G
*************************** 1. row ***************************
table: faar_homes
type: range
possible_keys: price
key: price
key_len: 5
ref: NULL
rows: 25
Extra: where used; Using index
