NO you have not left it too late, you have added some cost by not getting in earlier.
The starter pack will cost 10 usd After that you do not need to purchase any cards. You do need to get a starter pack, as game play does not activate unless your account has purchased a starter pack.
Once you get the pack you will need a few more cards yes. Buying these cards through booster packs can become expensive.
Some of the cards you will need will be available on the market. at a cheaper price than buying a booster pack, These are obviously the less valuable cards, but they are needed for some battles. A booster pack costs 2 usd. You get 5 cards in a pack, that is an average of 0.40c usd a card.
On the market some of these cards can be purchased at a lower cost. Some as low as 0.11c usd. The rarer cards and more desirable cards are more expensive and they do go up in price. You can gain a decent deck to begin with just 12 usd. Getting a starter pack and buying some loose cards off the market, Boosting one splinter.
The game itself does offer opportunities to gain extra cards through games using a ranking system and a quest system.
Yes, it's possible to play with just buying the starter pack.
You don't need to buy a lot of booster packs because there's a daily mission every day which awards you with 1 free booster pack every time you finish it.
So, players with starter pack can improve their deck without buying booster cards at all.
No, it's not too late to get in the game right now.
Make sure you have enough steem to buy some packs and once you have a summoner card along with 4 of the same group then you can start the battles. You need to watch some dtube videos to understand the basics but it is easy enough to master