Actually a lot of older movies are being put on to DVD that were not shot in HD. If you do your homework you can find movies on Blu-Ray that go back all the way to the 80s, which were most certainly not shot in HD.
It's just a matter of converting it over to Blu-Ray. Now will the picture Qaulity be as good as a movie like Transformers, no. However when they convert it over it will look better then it did when it Aired on TV.
As to why it hasn't been put out on Blu-ray it's due to various reasons. The first of which is Blu-ray is still rather new and most people don't own one (I happen to be one of those people, though im getting on in March). Once prices start to drop on blu-ray and more people own them then the flood gates will open and will see tons of old movies being put out on blu-ray. However for now it will mostly be popular shows and movies that have sold very well on DVD and they think will do well on Blu-ray, most being new shows and movies.
The other reason may be due to who owns the rights. However I believe Sony does own all the rights to Farscape, which means were likely to see them on Blu-ray in the next few years.
Also Farscape has much bigger fan base then Highlander: The Series does and there putting that on DVD, so that has nothing to do with it.
As for the cost, it won't be any more then the rest out there. The First season will go for about $50 dollars or so, maybe $80 because its a BBC show if I'm not mistaken, which generally means much higher price. So at most you might pay about $400 dollars.
Right now Blu-ray is still relatively new. The players them selves are rather expensive at the moment. So a lot of stuff will be new or popular shows and movies, as time goes on will see more obscure movies and shows.
I would love to see Stargate: SG-1/Atlantis on blu-Ray, along with Farscape, First Wave, Star Trek, Heck even the Star Wars Trilogy. I just have to be patient, they will come to blu-ray in time.