I've found propane price, varies from $2.70 to $3.10, mainly depending on, if state has sales tax or not and if it's included in displayed price. Depends by state and dealer. Level of service, also effects price. I always prefer, not dealing with grumpy person, that hates filling tanks.
I carry extra, so I just get it, when/where it's convenient. From high to low, you're talking couple bucks difference, per tank. It's not worth much inconvenience, to save couple bucks. I don't use exchange tanks, unless my tank is expiring. You get partially filled exchange, at double price, plus their often least convenient place/service.