Depends who you are playing against, or if you are in it to collect the minis.
In the frirst 5 sets it takes 2 to 3 cases to complete a set (with lots of dups obviously). The North Africa you can complete a set with 2 cases. If you are just looking for enough to play with some variety VS people who have a lot, one case of each and you are golden. If you are a new player starting out with the two latest larger-scale sets then 2 starter sets and 6 boosters each of 1939-1945 and North Africa and you can play.
eBay is your friend, as is doing some trading.
Over the years I’ve probably picked up 2 to 3 cases of each set, plus eBay, plus trading. And if you are looking for a ready-made collection with multiples of everything and have a grand you can have mine :evil:
To answer you question of is it an expensive hobby, to buy 1 case will cost you 80 bucks. I spend more than that on beer each month, so relatively speaking I guess its not bad. They release a new set about once every 9 months.