First off, most of Koh Phangan works on cash.

There are many ATM machines so you will be able to get cash out easily, just be mindful of your ATM fees. The yellow ATMs from Krunsgri Bank let you take out the most Baht at once (30,000 - which is ~$850). There are also many places to exchange currency on the island if you bring foreign currency with you, but the exchange rates aren’t as great as the ATMs.

Koh Phangan is considered a bit pricey for Thailand, but it is extremely cheap compared to the world.

For example:

• One of the most delicious meals on the island is a whole lamb shank baked in a Persian oven on rice with bread over it with tzatziki. This costs $18.
• A one-hour massage is $9.
• Beautiful, gorgeous, big, sea-view villas range between $150-400 per night for 4-8 people or ~$50 per night per person.
• A full on brunch from the best brunch spot on the island with eggs, salmon, bacon, tomato, coffee, toast, and orange juice is $17.
• The one tasting menu restaurant, which is a delicious 7/8 course meal and run by an Israeli couple, is $27.
• Dishes at local Thai restaurants are $2-9.
• Renting a scooter for a day is less than $7.

Just wanted to provide perspective.