To estimate a maximum purchase price for the coin in question, we can analyze the provided data:

1. Starting price: 90 euros.
2. Coin condition: 70 (MBC-), indicating an acceptable condition.
3. Auction history:
- In 2020, the coin was auctioned with a starting price of 30 euros and sold for 130 euros.
- In 2016, the starting price was 100 euros, with a final bid of 100 euros.
- In 2018, the starting price was 70 euros and sold for 130 euros.

4. Average final bid price: 120 euros, indicating that, in general, coins of this reference tend to sell for above their starting price.

Given that the coin has a condition of 70 and considering the history of final bid prices, we can make an estimation. The trend shows that, despite a starting price of 90 euros, the final bid price is usually significantly higher.

Based on the average final bid price of 120 euros and the fact that the coin has performed well in previous auctions, an estimated maximum purchase price could be around 120-130 euros, considering that the starting price is 90 euros and that the condition is acceptable.

Therefore, an estimated maximum purchase price could be 130 euros.