You can do this using conditional visibility. First, create 3 text fields in your CMS collection, 1 for single price and 2 for small/large price. Then simply create static text for small and large labels and set the conditional visibility to “if small price is set” and “if large price is set” to both the labels and the small/large price fields themselves. Also set the conditional visibility for the single price text to “if small price is not set”. This will mean that if you put in a single price, everything with the small/large price will not be visible and vice versa. I hope that makes sense.
More information here: http://forum.webflow.com/t/new-feature-conditional-visibility/22539