In my ECG starter pack (https://www.thegamecrafter.com/games/bella-elementorum-starter-pack) you get enough to make either 2 minimum size decks or one large deck.
What size would themed decks or expansion decks be, and how would the price stack up against the starter set?
So, assumming you're the maker of the game (btw the link listed is broken), there are a couple of things you could do.
1) if it's print & play like the starter pack that I found ($0.99) then I could see the price just staying the same.
if you're actually planning on going to print then it's a bit of a different story and I would say to either make the starter pack printable as well or find a way that the cards from the starter pack eventually make it into the themed/expansion packs.
2) going by what you've stated here, it sounds like the minimum a deck needs to be for a player is 30 correct or is it like M:tG? if it's like M:tG then a themed deck usually for them is 75 cards which is meant to have 15 of them be side board cards. depending on how you want the themes working possibly might influence how big the deck is going to be. Though note that most of M:tG's themed decks are from their expansions.
So this is as far as I can go with what I have available to me. If you'd like anymore help from me, send me a message, I'm almost always available. :D