Based on the data provided, we can observe that the coin in question has had an average auction price of 27.13 euros in previous auctions. Additionally, the current condition of the coin is 30 (BC+).

Considering that the coin will be auctioned in the coming days with a starting price of 6 euros and has had an average starting price of 20.34 euros, we can estimate that the final auction price could be around 27.13 euros, which is the historical average auction price.

Given that the condition of the coin is 30 (BC+), which is acceptable but not excellent, and considering that the coin has been sold in the past for prices ranging between 15 and 60 euros, we could estimate a maximum purchase price of around 35-40 euros.

Therefore, my estimation of the maximum purchase price for this coin in the next auction would be approximately 35-40 euros.