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 of 75 (MBC), which indicates that it is in relatively good shape, but not among the best qualities (FDC).

2. Previous Prices:
- In 2013, a coin with a condition of 90 sold for 11,254.63 euros.
- In 2014, a coin with a condition of 50 sold for 4,100 euros.
- In 2021, another coin with a condition of 90 sold for 6,952.57 euros.

3. Average Hammer Price: The average hammer price for this reference is 5,951.80 euros, which gives us an idea of the market value.

4. Current Starting Price: The current starting price is 900 euros, which is significantly lower than the previous hammer prices.

Given that the condition of the coin is 75, which is better than 50 but lower than 90, we can estimate that its value should be between the hammer prices of coins with conditions of 50 and 90.

We can make an approximate calculation considering that the price of a coin with a condition of 75 could be in an intermediate range. One approach could be to take a weighted average of the previous hammer prices, adjusting for condition:

- Hammer price for condition 90: 11,254.63 and 6,952.57 (average: 9,103.60)
- Hammer price for condition 50: 4,100

We can estimate that a coin with a condition of 75 could have an approximate value of:

\[
\text{Estimated Price} = \frac{(9,103.60 + 4,100)}{2} \approx 6,601.80
\]

However, since the average hammer price is 5,951.80 euros, and considering that the condition is not the best, a reasonable maximum purchase price could be slightly lower than this average.

Therefore, an estimated maximum purchase price for this coin could be around 6,000 to 7,000 euros.