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)
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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
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