I'm about 35 miles southwest of St.Louis just off I-44.
We've got two 500 gallon tanks, one for the garage and one for the house

We filled both of them in early January and it was $2.36 a gallon.

Around STL the propane company you use had a major effect on the price per gallon.
We use MFA propane (aka MFA Oil).

The one neighbor used Ferrell Gas and she always paid almost a dollar more per gallon for it.

Unless you own the tank you're locked in with the company that owns the tank, but most will supply the tank free if you switch to them.