To estimate a maximum purchase price for the coin that will be auctioned, we can consider several factors based on the provided data:

1. Starting price: 50 euros.
2. Average hammer price: 98.9 euros.
3. Hammer price history: Previous hammer prices varied, but there are several examples where the hammer price has exceeded 100 euros, especially in higher grades.
4. Condition: No information is provided about the condition of this coin, which can significantly affect its value. However, the average hammer price suggests that there is considerable interest in this coin.

Given that the average hammer price is 98.9 euros and considering that the starting price is 50 euros, a reasonable maximum purchase price could be slightly higher than the average hammer price, especially if we consider the possibility that the coin has an acceptable condition.

Estimated maximum purchase price: 120 euros.

This price allows for a margin for competition in the auction and is based on the historical interest in the coin.