We've been charged different prices for our refills in the last year. Our mugs say "refill offer subject to change or cancellation" under the prices, so I'm guessing that's why we are most often charged the current refill price. I don't push the issue.
Our mugs that we currently use are All-City, so they aren't that old. We do have one Christmas one. They have the skinny base that fits in car cup holders (and now our stroller cup holders).
The oldest one we have right now (not counting a couple of glass ones) is a Wildfire mug, but you can't close the spout; it's just an open hole, so we don't ever bring that one. I can't read the prices on that one anymore.