Some of them are, for example, influencer marketing might cost quite a bit. There are campaigns that are free but require more effort, too — like networking.
Some campaigns for apps that are already past launch need little to no investment on your part — your users will advertise your app through word-of-mouth for free if it’s good. Other times you can offer users referral bonuses. Paid advertising in other apps and on social media also might cost very little or a lot.