Prices in supermarkets are the same across the country, and they are easily 50% more expensive than in Thailand. Compare the prices for basic food, juices, milk, snacks etc.
A 20 year age difference is not that much to suspect her of being necessarily a gold digger. A lot of girls are naturally attracted to more mature and experienced guys. But if the age gap is 30-40 years (common theme in Thailand), you may start being suspicious.