The cost of a trip to Phuket, Krabi, and Koh Samui for a week can vary depending on various factors, including travel dates, accommodation choices, activities, dining preferences, and your travel style. However, I can provide you with a rough estimate to give you an idea of the potential costs involved.

1. Flights: The cost of flights will depend on your departure location, time of booking, and airline choice. It is advisable to check with airlines or travel websites for current prices. As a rough estimate, return flights from major cities to these destinations could range from $300 to $800 or more, depending on factors mentioned earlier.
2. Accommodation: The cost of accommodation can vary significantly based on your preferences, location, and type of accommodation. In popular tourist areas like Phuket, Krabi, and Koh Samui, you can find a range of options from budget guesthouses to luxury resorts. As a rough estimate, budget accommodations can start from $20 to $50 per night, mid-range hotels from $50 to $150 per night, and luxury resorts from $150 upwards per night.
3. Transportation: Getting around within these destinations can be done by various means, including taxis, tuk-tuks, or private transfers. Additionally, you might need transportation to travel between the islands. The costs will vary based on the distance, mode of transportation, and negotiation skills. A rough estimate for local transportation expenses could be around $20 to $50 per day.
4. Activities and Attractions: Phuket, Krabi, and Koh Samui offer a wide range of activities and attractions such as island hopping, snorkeling, diving, boat tours, national park visits, and cultural experiences. Prices will vary depending on the specific activities you choose. It is recommended to research and budget accordingly based on your interests. A rough estimate for activity expenses could be around $50 to $100 or more per day.
5. Food and Drinks: The cost of food and drinks can vary depending on your dining preferences. You can find a wide range of options from local street food to international cuisine in these destinations. Roughly budgeting around $15 to $30 per day for meals should give you a decent range.

Keep in mind that the above estimates are rough approximations and can vary based on personal preferences, travel season, and currency exchange rates. It’s always advisable to research and compare prices, book in advance, and consider any additional expenses such as travel insurance, visa fees (if applicable), and shopping.

To get a more accurate estimate, you can research specific costs for your travel dates, contact hotels and tour operators for pricing information, and consider consulting with travel agencies or experts who specialize in trips to Thailand for a tailored cost estimate based on your specific requirements.