To estimate a maximum purchase price for the coin in question, let's consider several factors:

1. Current starting price: 900 euros.
2. Current grade: 30 (BC+), which indicates a fairly low grade.
3. Auction history: The coin has been auctioned once before, with a starting price of 3000 euros and a hammer price of 5000 euros at a grade of 70. This suggests that the coin may have significant value in better condition.
4. Average starting and hammer price: The average starting price is 1950 euros and the average hammer price is 5000 euros, indicating that, in general, this coin has high sales potential.

Given that the current grade is quite low (30), the hammer price will not be as high as in the previous case (grade 70). However, the interest in the coin and its auction history suggest that there could be considerable value.

Taking all these factors into account, an estimated maximum purchase price could be in the range of 1000 to 1500 euros. This is based on the current starting price and the coin's potential, but adjusted for its low grade.

Therefore, a reasonable maximum purchase price could be 1200 euros.