Horizon have a fairly large selection of original rehersal scripts which are available as photocopies. Although it can be interesting to see how the rehersal version compared with the aired version ('Sarcophagus' for example, had quite a few changes), I think these are expensive for what they are. - There's a lot of white space which adds to the cost, presumably it was originally there for people to scribble additional notes and corrections on. You're paying over five pounds per script even before postage is added on.
If you just want a transcript of the episode to read through with friends or to check details for a story you are writing, then I'd download the relevent transcript from Lysator.
These transcripts were typed up by fans listening to the episodes - they are pretty accurate and I've found them to be immensely useful. In fact, to be honest, I find them more useful than the rehersal scripts as when writing fiction I need to know what was actually said on screen as opposed to what was there in the draft versions.
The most significant script Horizon have available that does not exist as a transcript is "Man of Iron". This is a 4th season script written by Paul Darrow that wasn't accepted by the BBC. If I'd been the BBC, I'd have rejected it too, on the other hand if you enjoyed Avon: a Terrible Aspect, then you might well enjoy this. If you want a story that presents Avon as superman having great fun shooting lots of robots with a pump action shotgun, then this is clearly just the script for you. Although Paul Darrow did a superb job acting Avon, I personally don't think he writes well for him. The character who works best for me in this script is Vila.
Bearing in mind the price, I can't recommend that you buy this, but you might want to borrow a copy out of curiosity.