Speakers are like anything else in Audio.
You get what you pay for.
A good set of Altec or Logitech gaming speakers will reproduce the same frequency range as a comparable set of audiophile stereo speakers or studio monitors.
It all depends on how picky you are, how much color you want in your bass and tweeter response and what type of music you wish to listen to.
For most people a set of mid range studio monitors in the $200 to $500 class will produce an extremely flat frequency response for most of the spectrum, but if you enjoy deep booming bass notes then you will still need to add a powered subwoofer to the mix.
Then again, if you purchase, let's say, a $100 pair of low end monitors from M-Audio like the AV32 model, while they still produce a decent sound, they are still comparable to say a Logitech Z263 2.1 computer speaker without the subwoofer.
My advice, if you're starting out, is to buy a set of good quality computer speakers and the proper adapter connectors/cables and hook it up to the LINE OUTS on your Korg to get you started. You don't have to go crazy. $100 to $200 range will get you some kickass speakers.
Should you decide you want better sound over the long run you could always upgrade to a set of Nearfield Monitors and appropriate subwoofer as time and money permit.
In the meantime, however, there's no urgent need for you to blow $1000 on speakers when you're just starting out.
Just to clarify, Computer Speakers and Home Stereo Equipment in general, are 100% electrically and acoustically compatible with keyboard instruments.
They all operate at line input levels and they are optimized for listening to music across the full range of human hearing.
What it ultimately boils down to is what the listener deems as acceptable quality for listening to his particular style of music. I'm not going to presume to tell you what style of music you should listen to.
Gary