Well, your questions are very market dependant (as in, geographical market). But I'll leave that be.
Essentially, there still is a price war going on. With the advent of cogent, and the near-chapter-9 NSP's, the price per meg for *large* commits is falling like a rock. Think we've pretty much seen the bottom (as in $50/meg easy from chapter 11 companies for 100mbit+ commits.. i.e. gblx, mnfx, etc.), and I think the bottom will rebound a bit as these companies either come out of restructuring or just die off alltogether. Expect the price of bandwidth to take a slight upward trend, not much, but instead of $50/meg for hugeass commits, you'll be looking at $75-100.
Essentially, everyone will stop trying to get market share and quickass revenue shots, and realize they they DO need money to keep their network in shape.
Most expensive transit? Easily uunet and sprint. Cable and Wireless may be coming in a close third, but I havn't sourced them lately.
Least expensive transit? It's probably a toss up, and depends greatly on your commitment levels. If sub 500mbit/sec, you're probably still looking at cogent. Over 500mbit, mfn probably is the most desperate.
Of course, this is all my experience and opinion talking. No real substantiated facts here. I by no means come close to the commitment levels needed here, but I do help source transit for those that do, and network regularly with engineers of many of the companies mentioned previously in the thread.
-Phil