How to assign a role to a user depending on order subtotal using Ubercart?
If you want to use that "Order subtotal" in a Rules "Condition", you could try to first add a Rules condition Entity has field (related to the field containing that "Order subtotal").

That's also what is mentioned in the Rules UI when you're adding a "data comparison" condition, somewhere it says:


  The data selector helps you drill down into the data available to Rules. To make entity fields appear in the data selector, you may have to use the condition 'entity has field' (or 'content is of type').


So if you don't have an appropriate "field" for it, then you may be able to get it to work also by using that 'content is of type'.

For a video tutorial that illustrates the importance of this Entity has field condition, refer to Data types and data selection, especially what is shown between about 13:30 and 17:30 in it.