I take those flights twice a month or so now....they are a lot cheaper if you are booking them AMS-BOS instead of BOS-AMS.

DL miles are easy to get, if you are smart about it. Last year DL ran a promotion where sharing miles earned a 100% bonus. It's only 1 CPM to share, and there was a 30k max per transaction + 30$ fee. This meant that 30k miles turned into 60k for 330$. Do this until you've reached the promo's limit and you've done well.

That being said, do take a look at the AMS-BOS pricing, and compare all three options, KL, AF, and DL before making a purchase. There's DUS and BRU too which are close enough to AMS. I usually pay around 500$ for a RT, and I've paid $430 recently (albeit through CDG).

That flight is good (in row 10 or 29).