To estimate a maximum purchase price for the coin in question, we can analyze the provided data, including starting prices, hammer prices, and the conditions of similar coins auctioned in the past.

1. Current starting price: 150 euros.
2. Average hammer price in the past: 123.03 euros.
3. Previous hammer prices:
- In recent auctions, hammer prices have varied significantly, reaching up to 500 euros in a March 2023 auction.
- Most hammer prices are in the range of 70 to 240 euros, depending on the condition.

4. Condition: There is no information about the condition of this coin, which may complicate the estimation. However, it can be observed that coins with higher conditions (above 80) tend to achieve higher hammer prices.

5. Price trends:
- Coins with a starting price of 150 euros have had hammer prices that vary, but generally, higher hammer prices have been seen in coins with conditions of 80 or more.
- In the case of the March 2023 auction, a coin with a starting price of 150 euros reached a hammer price of 500 euros, indicating that there is potential for a high hammer price.

### Estimation of the maximum purchase price:
Given that the starting price is 150 euros and considering that significantly higher hammer prices have been achieved in the past, a reasonable maximum purchase price could be in the range of 250 to 300 euros, assuming the coin has an acceptable condition and there is interest in the auction.

Therefore, an estimated maximum purchase price could be approximately 300 euros.