To estimate a maximum purchase price for the coin in question, we can consider several factors based on the provided data:

1. Starting price: 300 euros.
2. Current condition: 75 (MBC), which is a fairly good condition, although not the best.
3. Auction history:
- In 2011, the coin was auctioned with an estimated price of 698.26 euros and sold for 1617.02 euros, with a condition of 90.
- In 2015, the starting price was 384.3 euros, with a final price of 720.55 euros, although the condition is not specified.

4. Average final price: 1168.79 euros, which is significantly higher than the current starting price.

Given that the current condition of the coin is 75, which is lower than the previous auction conditions (90), it is reasonable to adjust the estimated price downward compared to previous final prices.

### Estimation of the maximum purchase price:
- Considering the average final price of 1168.79 euros and adjusting for condition, we could estimate that the maximum purchase price could be in a range between 50% and 70% of that price, given that the condition is not the best.

- Calculation:
- 50% of 1168.79 euros = 584.39 euros
- 70% of 1168.79 euros = 818.15 euros

Therefore, a reasonable estimated maximum purchase price for this coin, considering its condition and auction history, could be between 584 and 818 euros.

To be more conservative, you might consider a maximum purchase price of 800 euros.