If you are not desperate
Just figure out how much two days is worth to you, what they are asking from you is to make an investment in their company of €400+ for a chance for a job. If they aren't willing to make the same investment in you then what good is such a company? Does this company respect you as an employee the way you want to be respected?
If you are desperate
Even if you're desperate for a job you might be well served to stay away from a company like that, as it means that you will end up in a team of sub par developers. Only developers desperate for a job or straight out of school will apply for a job at such a company, which means that the entirety of a company likely consists of these kind of people. Now, some desperate developers willing to do these kind of tests will be actually great developers, but it's very likely that the majority of these developers will be inexperienced and/or fired from their previous jobs.
The risk of a scam
It's not impossible at all that they might give you production problems to solve as correctly noted by codenoire. This isn't necessarily the case, and they might actually give you a pure 'perfectly normal' (aside of the size) test, but this is extremely unlikely as a normal ethical company wouldn't ask for such an investment from a candidate, as this would result in losing out on the majority of qualified candidates.
Raise your concerns with them openly, and make it clear that you're willing to do a normal technical interview with one of their developers. That way both parties are equally invested in your application. If they are willing to dedicate a developer for 8 hours to sit with you for an 8 hour technical interview: go for it. And the same goes if they show in different ways that they are willing to invest in you, if they are willing to fly you over from another country to do a full day of testing in their headquarters: it's a bit over the top, but sure, that's worth considering. More than likely however when they are asked to make a similar investment they will quickly lose all interest in you (or if they are an actually ethical company change their recruitment process).
You might also want to consider reviewing their company on sites dedicated to this purpose like glassdoor.com to warn other potential employees. Thus helping both the rest of the community and other companies with more respectful recruitment processes. The danger of course being that they might try to retaliate, so whether you risk it is up to you, but it's definitely appreciated by the rest of us.