To estimate a maximum purchase price for the coin in question, we can analyze the provided data:

1. Starting price: 200 euros.
2. Current grade: 70 (MS-), which is a fairly good grade.
3. Auction history:
- In 2020, a coin with a grade of 30 sold for 150 euros.
- In 2022, another coin with a grade of 30 sold for 141 euros.
- In 2023, a coin with a grade of 70 sold for 300 euros.

4. Average prices:
- Average starting price: 138.75 euros.
- Average final price: 201.75 euros.

Given that the current grade of the coin is 70, which is the same as the last auction in 2023, and considering it sold for 300 euros, we can infer that the final price could be in a similar range or even higher, as the market may have changed.

Additionally, the starting price of 200 euros is reasonable, but given the sales history and the grade, an estimated maximum purchase price could be in the range of 250 to 300 euros, considering that the previous final price was 300 euros.

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