Congrats on your trip to Europe, Meghan! When I was in France two summer ago, I took the SNCF train from Paris to Lyon and back, and it cost me $200. I thought that was very expensive, but since I was only going to those two cities in France, it was cheaper to buy one isolated ticket. So the unlimited pass doesn’t seem quite so exorbitant in comparison. If you’re going to be traveling for a while and going from country to country and city to city, a Eurail pass, even the expensive one is cheaper than last minute flights. Even considering the budget airlines, since they will gouge you on checked luggage.
If you decide to travel via air and train without the pass, it will take considerable organization and price comparison to ensure that you are getting a better deal. That being said, I have never bought the unlimited pass, so I cannot speak from experience; however, if you can afford it, it gives a peace of mind that you’ve already paid for all your travel and you can go where you want whenever you want.
You can also try blablacar if you are open to ride shares. It could be a great way to meet new people, and it’s certainly a spontaneous way to travel, if you are comfortable traveling with strangers.