The function requires 3 parameters:

1. Natural Number
2. Nearest Power of 10
3. Direction

Simple Use Cases:

SELECT * FROM dbo.udf_RoundNearest (1264,10,0) /* The Direction 0 indicates the nearest Power of 10 round down. Result = 1260 */
SELECT * FROM dbo.udf_RoundNearest (126412,1000,0) /* Result 126000*/
SELECT * FROM dbo.udf_RoundNearest (126412,1000,1) /* Result 127000*/

Integrating the function with a table/view/derived table/CTE:

;with tmp (Value) as
  (select 1236 union all select 6584 union all select 9999)
    select t.*, fn.Number
    from tmp t
    cross apply dbo.udf_RoundNearest (Value,100,0) fn
/* Value Number
   1236 1200
   6584 6500
   9999 9900 */