To estimate a maximum purchase price for the coin in question, we can analyze the provided data, especially the previous auction prices and the current starting price.

1. Current starting price: 350 euros.
2. Auction price history:
- Previous auction prices varied significantly, but there are some standout auction prices:
- 650 euros (2019-05-23, condition 95)
- 480 euros (2018-10-17, condition 100)
- 450 euros (2019-12-11, condition 95)
- 350 euros (2019-10-17, condition 95)
- 270 euros (2019-10-17, condition 95)
- 220 euros (2022-10-19, condition 90)
- 260 euros (2023-03-16, condition 95)
- 280 euros (2023-03-01, condition 95)

3. Condition analysis: The condition of the current coin is not specified, which complicates the estimation somewhat. However, by observing the auction prices of coins with high conditions (95 and 100), we can infer that the auction price could be significantly higher if the condition is good.

4. Maximum purchase price estimation:
- Given that the starting price is 350 euros and considering that in the past auction prices have reached up to 650 euros for high-condition coins, a reasonable maximum purchase price could be in the range of 400 to 500 euros, assuming the coin has an acceptable condition.

Therefore, an estimated maximum purchase price could be approximately 500 euros. This is based on the auction price history and the current starting price.