It depends on your age, the AXA policy for OA visa is around 7 to 12K for someone in early/mid 50’s depending on your choice of deductible, for someone in late 60’s between 20 and 40K. You can get a price online directly from AXA, here is the link, no need to go through a broker.
the insurance requirement is $100K or 3 million baht. You can use a non Thai insurance policy but you need to get a letter from your insurance company clarifying the amount and many overseas insurance companies don’t like doing it. Some of my clients have told me they were declined due to the maximum coverage amount not being in USD$. To make it simple I would recommend you buy directly online with AXA Thailand who are one of the approved suppliers, no need to go through an agent, here is the link.