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

1. Current Condition: The coin has a condition rating of 80 (MBC+), indicating that it is in fairly good shape, but not the best possible condition.

2. Auction History:
- The average hammer price for similar coins has been 521.43 euros.
- In previous auctions, hammer prices have varied, but there are examples of sales that reached up to 900 euros (in the case of a condition rating of 95).
- The last auction with a condition rating of 80 had a hammer price of 550 euros.

3. Starting Price and Average Starting Price:
- The current starting price is 250 euros, which is below the average starting price of 326.79 euros. This suggests that there is potential for the hammer price to be higher.

4. Sales Trends: The coin has been sold in 14 out of the last 15 auctions, indicating good demand.

Based on these factors, an estimated maximum purchase price could be in the range of 600 to 700 euros, considering that the condition is good and that the hammer price history for similar conditions has been favorable.

Therefore, a reasonable maximum purchase price could be 650 euros.