Does the value you get of the game change depending on which time of the year you buy it?

Actually, the only change is up, as the game was improving and expanding pretty much constantly from the first early release to version 1.1. And it value is going up, when you buy in early access you're only getting the current (unfinished but playable) state and a "promise" that it will get better in the future. When you buy the finished product you're already certainly getting that better state, so it makes sense that it's more expensive.