How much cash do i need to obtain the white flute in Pokemon emerald?

You don't need any cash to obtain the White Flute in Pokémon Emerald. It's a key item you get for free, not purchased.

Here's how to get it:

1. Beat the Elite Four and become Champion.

2. Speak to Mr. Stone in Slateport City. He will give you the White Flute.

The White Flute is used to wake up the legendary Pokémon Latias and Latios, which can be found in the game after you become Champion.