I'm not sure where you shop, but every store I go to sells them for the same price. Actually, I've found much better deals on +r format that -r.
The price difference is getting less and less, but I know what you are asking. The reason is mostly due to volume. The -R format is the one that is recognized by the DVD Forum (the people who decide what is and isn't a DVD.) It is also useable in more types of burners than +R. The most notable is stand alone DVD recorders. (sort of like a VCR, but uses discs rather than tapes.) As more people use -R, the price goes down. A good example of this is the cost of video tapes. I can buy VHS tapes for about $1 each, but Beta tapes cost about $10 each still. This is because for every 1000 VHS tapes sold there's probably only 1 Beta tape sold. Now many people say that +R is superior, and I've seen secifications to back this up. Myself, I know neither of my stand alone players will play +R. To me this makes the -R format superior.
I haven't seen that at all. Are you in the US or europe? I've seen the +R drives that write at 4x are cheaper but not the media. What's the difference between the two?