To estimate a maximum purchase price for the coin that is going to be auctioned, we will consider several factors, including the starting price, the condition of the coin, and the hammer prices of previous auctions for coins with similar conditions.

1. Starting price: 50 euros.
2. Condition of the coin: 90 (EF).
3. Previous hammer prices:
- Coins with a condition of 90 have been sold in the past at varying prices, but some examples are:
- 60 euros (2020-12-14)
- 55 euros (2020-12-14)
- 42 euros (2020-07-14)
- 70 euros (2020-07-08)
- 62 euros (2020-11-03)
- 76 euros (2020-11-03)
- 88 euros (2020-11-03)
- 190 euros (2020-12-14) for a coin with a condition of 80.

4. Price analysis:
- The average hammer price for coins in similar condition seems to be in the range of 55 to 70 euros, with some exceptional cases reaching up to 190 euros, but this may depend on the demand and rarity of the coin.
- Given that the starting price is 50 euros, and considering that the condition is good (90), it is reasonable to expect that the hammer price could be above the starting price.

5. Estimation of the maximum purchase price:
- Based on the previous analysis, a reasonable maximum purchase price could be between 80 and 100 euros, considering that the condition is good and there is interest in the coin.

Therefore, the estimated maximum purchase price for this coin could be 100 euros.