If you are a foreigner, you are 100% always going to be quoted a foreigner price that maybe at least 3X the regular everyday price when trying to buy things from the locals.
You need to understand this.
The actual selling price is going to be up to 2 things?
How are your negotiating skills and the ability to walk away without over paying.
You should learn some very simple Thai prices to negotiate a lower or acceptable price. Can you do 100 baht? The vendor will counter so understand this is a cat and mouse game of back and forth; then vendor wants to win always.
You should also understand many, many vendors sell the exact same merchandise.
If you DO NOT get a price acceptable to you, move on to the next vendor.
Often the original vendor will chase after you and give you a much better price.
Unfortunately, that is the way it is in Thailand.
These vendors are trying to make a living the only way they know how.