While it's true that your cache size won't be that large if your area of interest is rather small, tile generation at the largest scales can still be quite heavy. That is where the cost of tile generation and storage can really get you. However, credit consumption for tile storage isn't nearly as expensive as the cost for tile generation. My recommendation would be to generate your tile cache on a local machine and then upload a tile package to ArcGIS Online. This will bypass the tile generation credit costs and also allows you to only cache tiles for the areas where you have data, so that you aren't generating a bunch of blank tiles. However, I haven't figured out yet how to add a generic "blank" tile to the ArcGIS Online tile cache to be used where there is no exisiting tile. So when the tiled service is used in an application, the javaScript console returns 404 errors for each missing tile. It doesn't affect the end user, but I am not sure whether it slows performance of the application at all.
That being said, I'm not sure how to access the number of tiles on an ArcGIS Server tiled service.
