Why do you want to run an instance for that many people? Do you have 1000 people already, or is that just a number that you decided to work with? Why would people be interested in joining an instance with less functionality than the bigger ones?

Also: does it have to be Mastodon, or would you consider other alternatives? Pleroma and GoToSocial would be a lot easier to manage.

https://masto.host/

To give you ballpark figures. Self-hosting will not save you much compared to that.

Software like Akkoma or GoToSocial will save you some CPU and RAM costs, but are more optimized for smaller instances.

Storage costs should be pretty low for an instance very few would want to use.

10 to 50 dollars, 400 GB, upgrade later if need be with other services, I think 4gb of ram is enough

Speaking of mastodon, any recommendations on lightweight & featureful Android apps for Mastodon ?? (I understand that this may not be the right post to ask, but still....)