I'm not sure I'm satisfied that this particulary mystery has been answered. OK, so there was a tremendous amount of sentimental value with this toy plane. Was it really worth setting up an elaborate bank robbing scheme to retreive it? People's lives were put in jeopardy with loaded weapons. Not to mention another crime to add to her rap sheet. Then, all she went through on the island to get it back? Lieing to Jack, fighting with Sawyer? Is this all there is to this plane? I think either there was something more in that envelope besides the plane, or we are missing something. Sentiment is all well and good, but her attachment to the plane has not been well explained to me.
Also, as some have suggested elsewhere...how did this plane get from the back of Tom's car into a safe deposit box in New Mexico...a safe deposit box Kate had the key to, but was not a signature on, so she didn't put it there it would seem. Many questions...so few answers.