I think that's the deal breaker for me. If I order $35 worth of stuff I already get free 2-day shipping. I use Prime because I don't have to think about order totals or shipping - it just shows up the next day.
The problem is a ton of items Amazon sells aren't Prime eligible. And that seems to include any lower prices items. Then you're back to getting a cart minimum, which at that point you might as well drop Prime entirely.