Well if you already have a decent laptop and some good piano soft synths, you can get a weighted 88-key controller for around 200 bucks. And I noticed you were asking about controllers (MIDI). Basically they transmit no sound data, only messages containing note number, velocity, possibly aftertouch, etc. A soft synth or DAW (pro tools, cubase, fl studio, etc) interprets these signals and uses them appropriately - usually to play the corresponding note on a soft synth, but you can actually control your daw using midi keyboard shortcuts the same way as a computer keyboard shortcut. It's a good thing to have for computer-based musicians, and most synths and even cheap consumer keyboards have midi I/O on them, so it's very accessible.
Another cost in addition to a good laptop or computer and a soft synth may be a MIDI interface for the computer. I use a midiman (now bought out by m-audio) midisport 2x2 usb interface. This allows me to run the midi cables from my keyboard into the interface, then into the computer. Very convenient. Musician's friend has the old original midiman version still at
http://www.musiciansfriend.com/product/MAudio-USB-MIDIsport-2x2-MIDI-Interface?sku=701226
However, m-audio has apparently since bought out midiman, so they reworked the case design and raised the price a bit
http://www.m-audio.com/products/en_us/MIDISPORT2x2-main.html. I'd go with the former route if you needed an interface, but there are cheaper options available (search for midisport 1x1)
Another thing, as much as I love Korg products, if you do consider the microKorg down the road, I would highly reccomend looking into the Alesis Micron first. It's the same price, but packs quite a bit more synth (including 3 oscillators instead of 2 and full sized keys instead of mini)
Hope this helps!
EDIT: Here's some links...
afformentioned portables:
Korg MicroKorg
Alesis Micron
and their big brothers:
Korg MS2000B
Alesis Ion