Well, there are some cheap alternatives. In fact, the stuff I use is very minimal. I love free software.
Here's how you can do it free: (or much less expensive)
Soundfonts: Use Megafont, a free program that runs soundfonts. Either that or Synthedit, another free program that has the ability to run soundfonts and VST instruments and all sorts of cool stuff.
Sequencer: Freeware sequencer. There are many decent free sequencers available. I just can't think of any off the top of my head. I think Cubase VST is free, though personally I hate Cubase.
Synthesizer: Well, there's really not too many free synthesizers out there.
Go by a $20 casio (ugh) synthesizer with midi in and out ports. You will not be using it for the sounds anyways, just the keyboard.
If you are ever looking for a good keyboard with awesome sounds and realistic keyboard feel, get the FP-3!
EDIT: By the way, this is a song I just made that I would've entered in this contest if Mods hadn't outlawed pianos. http://www.sonic.net/~schlae/herculeaneffort/realsolo.mp3. It's sorta a demo of the soundfonts I made that I will put on my upcoming soundfont site. Enjoy the song and please give me comments on the soundfonts.
Oh, soundfonts, soundfonts, soundfonts...
Here's how you can do it free: (or much less expensive)
Soundfonts: Use Megafont, a free program that runs soundfonts. Either that or Synthedit, another free program that has the ability to run soundfonts and VST instruments and all sorts of cool stuff.
Sequencer: Freeware sequencer. There are many decent free sequencers available. I just can't think of any off the top of my head. I think Cubase VST is free, though personally I hate Cubase.
Synthesizer: Well, there's really not too many free synthesizers out there.

If you are ever looking for a good keyboard with awesome sounds and realistic keyboard feel, get the FP-3!
EDIT: By the way, this is a song I just made that I would've entered in this contest if Mods hadn't outlawed pianos. http://www.sonic.net/~schlae/herculeaneffort/realsolo.mp3. It's sorta a demo of the soundfonts I made that I will put on my upcoming soundfont site. Enjoy the song and please give me comments on the soundfonts.
Oh, soundfonts, soundfonts, soundfonts...