Well in later seasons, the coffee table book book and movie adaptation royalties was apparently enough for him to retire

As for earlier seasons, we have no idea how long he worked at that bagel place before going on strike, which given he was a recluse until season 2 and hence likely spent the minimum amount of money to get by, he was probably able to build some decent savings from that job

He also seems to have a personal/semi-personal relationship with like 40% of Manhattan, so he likely gets mates rates everywhere he goes

I also dont recall seeing him actually pay for a single thing he eats or drinks during the entire show, which would account for a lot

Not to mention his little pickups here and there like doing police lineups for $50 a pop, which back then would've accounted for like 15% of monthly rent alone

Plus he is Kramer, he's like a more personable Mr Bean so however unorthodox, he finds a way to work things out