Given the funky extras / stretch goal math, I am looking at this breakdown:
Campaign Total: 6.699€
Amount of Claimed Rewards: 3.568€
Total of Extras: 3.131€
If roughly half the extras will be giving 50% toward the stretch goals, that would put us at around 4.350€ toward stretch goals. That would get us a good way toward stretch goal #1. If all extras are slipcases, t-shirts, dice sets and ref screens, then we are only halfway to most stretch goals.
Average backers are putting 42,70€ toward the books themselves. If we assume 25% of the extra amounts are going to stretch goals, one backer is around 50€ toward stretch goals. There is a total of 20.100€ of stretch goals assuming 3 module reprints, 1 Full Color Section and 1 Maria Appreciation Fund. So that's 318 average backers that we'd need to get for absolutely everything. That's not outside the realm of the achievable.