That is extremely overpriced. It is more labor intensive for working abroad, but not by that much. But if you were paying someone in the Netherlands to do your US tax return, that might explain it. Next year, get a US tax preparer and I can't imagine it being more than $200-$300.

would it be hard to do it by yourself?

Yes and no. You have to do a 1040 no matter what and you have to include a number of special forms that you must get absolutely right or you deal with alot of BS from having to talk to the IRS. And that's just for a simple tax situation.

If you have income from multiple sources, some of which may be in the U.S. and some not, even if it's as simple sideline business, then it gets more complicated as you also have to calculate profit/loss for that as if it were in the U.S., then subtract your taxes paid in your country, and demonstrate that it's all below a certain threshold. And there's also payroll tax exemptions, etc.