My mother was better about refraining from voicing her heartbreak directly to me, but she certainly made no secret of the fact that she felt the same way your mom does. Can't say I blame her, but I agree that it's *not* something you need to hear right now.
When we moved over here last year, we actually used FF miles to bring my mother with us for the first week. She was invaluable in keeping an eye on our kids (2 and 4 at the time) while we sorted out details and got ourselves generally organized, which made an enormous difference to us. In return, she felt reassured that she'd still remain a part of our lives even while we were living abroad, and being able to picture where we were and what we were doing helped her a lot when she returned home. I know not everyone has this kind of miles (we just happened to have 10 years of my husband's miles from trips back and forth to London), but if you do, it might be something to consider.
In addition, we make sure to talk to my parents every weekend on webcams (we use Skype). It's not the same as being together in person, but the fact that my kids can be silly and natural with their grandparents and the fact that my folks get to notice every haircut and see every cool art project helps everyone feel like the relationship is as strong as ever. Perhaps even if the airline ticket idea is out of reach, you could buy your parents a webcam as a "goodbye" gift? I suspect your mom just needs a little reassurance right now...