Original Tron film

[RT/M]

I have been looking to buy the original Tron film on DVD it ranges from £23 to £30+ any ideas why, its an old film i thought it would have been about a tenner at the most,i seen it years ago but i want to get it for my son to see, so when i take him to see the new Tron film out soon he will understand what its all about.
I`m still going to get the original no matter what, not to sure where from yet