Which cheap plywood are suitable for use as siding-and-sheathing?
While CDX (rated Exposure I) has exterior glue, it is not intended for prolonged weather exposure. You should use a panel that has a true exterior rating, even if it is rated for sheathing and not siding. Try looking for APA Rated Sheathing C-D Exterior plywood. You can use any panel thickness with a span rating for 16" spaced studs, but with 24" studs, use a panel with a roof span rating of 24 or more. That will be as cheap as you can go and still have reasonable life expectancy.