Based on the provided data, we know that the coin has a starting price of 30 euros and has been auctioned once in the past, selling for an average closing price of 55 euros. The coin's condition is described as "BC+".

Given that the coin has been sold once before and the average closing price was 55 euros, we could consider this price as a reference to establish an estimated maximum purchase price. However, keep in mind that the coin's condition is an important factor to consider.

Considering that the coin's condition is "BC+" (suggesting acceptable but not excellent preservation), and taking into account the previous average closing price of 55 euros, we could estimate a maximum purchase price around 60-65 euros. This is assuming the coin is in a similar condition to the previous auction.

It's important to note that the final auction price may vary depending on factors such as demand at that specific time, competition from other bidders, and buyers' subjective perception of the coin's value in that condition.