I think that's way too hard to answer since it depends on so many factors. But again, my experience isn't that the pay is too low. It's not like there are 10 people coming to the interview with no one taking the offer because of the salary. It's more like no one have applied for the job in three months and the external recruiters sniping LinkedIn didn't find anyone.
But to give some kind of estimate just so you know what page I'm at when I say they pay well I would say somewhere between $60-75k.
So I checked on glassdoor, which is known to lowball tech wages, for Washington D.C. So It isn't NY or Silicon Valley/Forest. The Median wage for a System engineer is 182K USD. At the current conversion rate that is 167.7K Euro.
Unfortunately, that's your problem.
But to give some kind of estimate just so you know what page I'm at when I say they pay well I would say somewhere between $60-75k.