How do I select the most recently created entity item in Rules
If one 'price entity' is created based on the daily prices per day, you could use the 'after saving new content' action, restricted to your price entity type (provided you entity can be seen as a node.)  Then, calculate the values for your products at that time, using the values within the newly created price entity.

Otherwise, you could put a 'last entry' field on you entity and use another rule to set this field to 'true' on a newly created entity and 'false' on all other entities.  Then, set a condition on the original rule that filters entities based on this field.