To estimate a maximum purchase price for the coin in question, we will consider several factors:

1. Starting price: 200 euros.
2. Auction history: The coin has been auctioned 5 times, of which it has sold 4 times. This indicates a consistent interest in the coin.
3. Average starting and hammer price: The average starting price is 308.55 euros and the average hammer price is 504.78 euros. This suggests that, in general, coins of this reference tend to sell for a price higher than the starting price.
4. Condition: The condition of this coin is 70 (VF-), which is an important factor. Coins in better condition tend to reach higher prices.
5. Previous prices: Previous hammer prices varied, but the highest was 700 euros in 2017, indicating that there is significant potential to reach high prices depending on demand.

Given that the condition of this coin is 70, which is an acceptable but not excellent state, and considering the average hammer price of 504.78 euros, an estimated maximum purchase price could be set in a range between the average hammer price and a bit lower, since the condition is not the best.

Therefore, an estimated maximum purchase price could be around 450 euros. This reflects a balance between the historical interest in the coin and its current condition.