One way to Europe

Did you try doing a multi city destination search on something like google flights or Expedia? Not sure what you are looking for but no the flights shouldn't be more. Generally you can fly into one city and out of the next for roughly the same price.

Personally I'd book your flights ahead of time, why have that on your mind and unless you are going for 3 months or more it wouldn't make sense financially to book it while you are over there? if you plan on booking your flight literally within days of flying out then yes it's going to be much more expensive, so that removes any motivation to 'stay flexible'.

Yes, I booked $399 flights to Dublin with $80 extensions to London. My father in law paid $1250 for flights straight to London 2 weeks before that. I wasn't planning on going but I set some flight trackers to London, Dublin, and I think Paris and those low rates popped up for about a 24 hour window so I jumped. You need to set those well ahead of time, and know what you are willing to pay. Then if a good deal pops up be ready to jump. Unless you are getting direct flights from the states, I would say a 'good deal' is generally anything under $1000/ round trip. If you get something under $800 definitely, under $600 is very rare and the $400 fares some of us got are blue moons.

Thanks to both for the info. Not sure why I was seeing what I was seeing in the delta app.

Am definitely buying in advance. And will set a flight tracker once I figure out how....

Anyway, I appreciate the feedback.