Answers
The whole scheme of the Golden Visa process requires real estate agents, developers or individual owners from the one side and a strong consultancy team from legal and tax aspects from the other, in order to be compliant with all the applicable provisions of the related laws. In order to ensure an successful implementation of your investment endeavor, it would be advisable to address your case in a consultancy company, acting as a one-stop-shop and providing all the above services through its team of professionals. As per the market prices besides the official tax prices by the government and average market prices' index issued by the Bank of Greece, you could compare the offered prices in same areas for similar properties in the existing real estate platforms in order to have a first picture of the market.
There is no official list for prices. The best solution is always to choose the best local partners in order to avoid any bad implications.