This is for the Android App? How much are the expansions? It says season pass? Is that forever? Or just a year?
Let's help you out with some useful information:
The "standard" expansion packs (Rook City, Infernal Relics, Shattered Timelines, and Wrath of the Cosmos) are priced at $5 each. The mini-packs have one mini-expansion hero, villain, and environment (Unity, Ambuscade, etc) each, and are priced at $2. IIRC, the pricing of Season 1, which has all of the previously mentioned standard expansions plus 3 mini-packs worth of content, is $25.00 (representing a small discount vs individual purchases).
Season 2 has just started rolling out content, and there's going to be LOTS of content there. I think the pricing is currently $25, eventually rising to $30 after all of the planned content is out. What I can vouch for is that Vengeance has just dropped (yesterday!) at a stand-alone price of $8.00.
Once you buy the expansion, it's yours - the seasons refer to development cycles and stuff that was planned all at the same time.
Does that tell you want you wanted to know?