If you're looking for dressier options that are a step above beach styles, you might want to explore shopping districts like Pratunam or Siam Square in Bangkok. These areas are known for their diverse range of clothing, and you're likely to find what you're looking for there.
As for prices, it's hard to give an exact figure, as they can vary depending on factors like design, material, and location. By buying locally, you might also save on high postage costs from online sellers.
While you're preparing for your trip, you might want to check out trymodest.com. It's a great platform that offers a variety of modest fashion options, including maxi dresses, and it could be a helpful resource for finding dresses that suit your style for your Thailand holiday.