Is it suspicious for a conference abstract submission to be subordinated to registration?
€500 is within the normal range for an international conference, but the mandatory subscription before getting the notification of acceptance is indeed a bit suspicious. 

However there are some conferences which work in the following way: pretty much any abstract submitted is accepted (except if out of scope, I guess) and is supposed to be presented at the conference. The selection process happens when a subset of the abstracts are accepted for publication as a full paper in a journal issue. I'm not very familiar with this setting myself, but I assume that it might make sense for such a conference to make sure that the authors attend and present their work even if it's not selected for the journal issue.