Yes if you buy the latest expansion you will get all previous expansions. The latest expansion is Battle for Azeroth. It has been out for over a year. It costs $63 Canadian for the standard edition. (roughy $45 usd). The deluxe edition might have some cosmetic things like a pet or a mount but you don't really need, which costs more. Sometime this year 2020 (No specific release date : Could be summer - fall) a new expansion will come called "Shadowlands"and you will have to pay for that expansion as well if you want the latest content, which may cost another $40 usd - Or you can buy it now for $40 usd(prepay).
You also need a subscription to play monthly. It is roughly $10-15 usd per month, you get discounts if you buy 3+ month subscription at a time.
You can try out the game for free up to level 20 by making an account and selecting free trial.
This post was edited by silentscream on Apr 16 2020 02:59pm