If you have created a new Story Map Crowdsource app through the online web builder, the only credit consumption should be the feature service storage. See this chart for specific pricing: ArcGIS Online | Service Credits. Each contribution is a new record in the feature service which is the first row in the table "Feature Services Storage." All photos are uploaded as attachments to the feature service storage and is calculated at the lower "Data Storage" rate. The photos are optimized in the builder before they are uploaded so each contribution should only be about 500kb of attachments per contribution. For a better estimate based on the number or records, data storage, additional map layers that may be added, you should contact your customer sales representative.
