Remember that examples are always made to demonstrate the strengths and hide the weaknesses of any piece of software. And with music sequencers, they may be using external sample libraries that you'll have to pay more money for.
Try out as many of these demos as you can until you find something that you are comfortable working with. I'm using Logic Studio 8 right now, because I tried out the sequencer alone, and really liked the interface for drawing in automation, as well as the MaxMSP-style Environment window for routing audio and midi data.
Another thing to ask yourself is what styles do you see yourself composing in?
Here's some sequencers and sequencer-based DAWs(digital audio workstation):
FL Studio has already been mentioned, and is probably going to get you the most bang for your buck in terms of included instruments and effects.
Reaper($60) - Sort of the inexpensive alternative to those $600 programs you mentioned (only $60). This is also the software that will have to be used when producing tracks for Harmonix's upcoming Rock Band Network. The demo also never expires, so it's possbile to get a real feel for it before you buy.
Sonar Home Studio($139, or $209 for the XL version which comes with a bunch of instruments, effects and tools) - Is a more affordable, less-powerful version of Cakewalk's massive Sonar DAW.
Sequel 2($99) and
Cubase Essential 5($149) are two of Steinberg's entry-level products for music composition and production.
Here's some trackers and tracker-based DAWs - they're similar to a typical sequencer, but instead of a piano roll view you get a list of note events, which can be easier and faster for some to interpret and create. I've just started learning to work with these since I come from a traditional musical background, but I find they are typically more suited toward electronic composition with a beat. More fluid styles of composition are possible but personally I don't find these to be as efficient for that purpose:
Renoise - I believe this is the only one that costs anything, $75
ModPlug
Jeskola Buzz
MadTracker
Psycle
Skale Tracker
Others:
Anvil Studio - I put this below here because it really is its own beast, I never used it much myself but it's a very capable midi sequencer/notation program.
As you get more into producing music on your computer, you'll find that more of your money is spent on sample libraries than anything else. Download as many trials of these and other products as you can and see what you find yourself to be most productive in.
[EDIT] Wow, I didn't realize Kellog sued Image-Line because of Froot Loops