What visa are you having now? Tourist visa? If you have a non B or O visa, no additional charge to open an account.
If you are on tourist visa, they are selling you an insurance in order to process a bank account for you. If you really need a Thai bank account then just get the insurance that the bank provide. Bank will not scam your money as there is paper work involved in any money transaction.
How many times do you plan to come to Thai in a year and how long you plan to stay per entry. If you stay 28 days, go out and come back in again maybe 2 times, you might get a red flag and immigration will question you.