To estimate a maximum purchase price for the coin that will be auctioned, we can analyze the provided data, especially previous auction prices and the current starting price.

1. Current starting price: 500 euros.
2. Average hammer price in the past: 655 euros.
3. Previous hammer prices:
- Hammer prices have varied, with some reaching up to 1200 euros in higher conservation conditions (such as 95 or 100).
- Most hammer prices are between 500 and 900 euros, depending on the conservation.

4. Conservation of the coin: There is no information about the conservation of this coin, which makes the estimation more uncertain. However, based on the hammer prices of coins with similar conservations (some with conservation of 80 and 85), we can infer that the hammer price could be in a similar range.

5. Analysis of previous prices:
- In previous auctions, coins with similar starting prices have had hammer prices ranging from 500 to 900 euros, depending on the conservation.
- The trend shows that the hammer price can be significantly higher than the starting price.

### Estimation of the maximum purchase price:
Given that the average hammer price is 655 euros and considering that the starting price is 500 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 acceptable conservation.

Estimated maximum purchase price: 800 euros.

This price allows for a margin for competition in the auction and aligns with previous hammer prices for coins with similar characteristics.