I think that you definitely need to let your guests know how much to budget for the trip. I didn't include it on the invitations but I did include the website link on the STD and included our Travel Agent's info as well as the info for the resort and room prices. That way our guests could figure out the cost for their trip and save up for it if money is tight.
Airfare is trickier since if you want to do a group purchase for airfare, it has to be from the same airport, same exact flights, same dates.. Basically for us it was impossible since we had guests coming on all different days from different cities and for a different number of days. Plus I did NOT want to be in charge of everyone's airfare because if there were any issues, I'd feel responsible for straightening it all out and I was stressed out enough with the wedding.
I did research flight costs and include them on the website (approximate $$) which obviously varied depending on airline, day, time, etc. I also had a guest who waited until the last minute to buy the airfare (one of my BMs!!!) and her airfare cost was twice as much as others so I would suggest that they do not wait until the last minute as prices tend to go up when you get closer to the date.