It depends. If you like to run a pool for educational/technical reasons, you don’t need any stake on testnet. On mainnet, you need 500 ADA for the pool deposit. Your pool then needs to attract delegators - it approx. needs 1m ADA to win the block lottery ones in every epoch (i.e. 1 block / 5 days). 100k will give you 1 block every 50 days and so on…
This has been explored and compared to “just delegate” to some extend in this post.