Depends on where you buy them. If you go to Radioshack you should be able to get decent ones for a total of just under $30. While you're there, get a couple of these:

stereo RCA->RCA cables should be easy to find in someone's junk drawer, and put together with those adapters, you'll be set to plug into either a DI box OR a DJ mixer.

thank you for correcting so these cables wont cost me over 30 $ right?

DAMN. I meant THREE 1/8" to RCA if you're not prosounded.

you have helped me so much dude!!!!! thank you. so i will be on my way today buying cables

The way I've always seen it, anything before the mixer/kp is your problem, so if you want a DI in there somewhere it's yours to buy. But anything after that point is the sound guy's problem. For your particular needs, a mixer will do fine and 99% of all sound techs are capable of dealing with a line level rca output. Most venues/bars that have DJ nights are already setup this way, and if you play a more traditional stage, the sound techs will have tons of little DIs on hand should the need for one arise.

That mixer however will probably add some noise and hum to your signal. I used to have the "realistic" version of this a long time ago, and the AC would bleed into my signal all the time. You don't notice it to much at home, but when plugged into a large sound system...it's damn annoying. You have to remember, Balanced signals basically just mean that your cables won't pickup interference between the balanced output source, and balanced input destination. Any noise/interference/goblins you have in your signal before converting to balanced will remain.

And finally, for your specific setup and cables... it depends on how you plan to make your music. This is a 4 mono channels mixer so if you want 3 gameboys in there at the same time, at least two of them are gonna have to output in mono (ie, no panning effects). You could use channels 1/2 for two mono gameboys, and channel 3/4 for a stereo one. Then take the ouput of your mixer, violently send that to the KP and give the KP's output to the sound tech.

If your GBs are prosounded, then you need 2 stereo RCA cables to plug all three in the mixer (two in mono sharing a cable, one in stereo using its own). If they're not prosounded, then use two 1/8" stereo jacks to stereo RCA to hook them up to the mixer. Then you'll need a stereo RCA cable for sending the mixer into the KP and then one other stereo RCA to send the KP's output to the sound tech. To record your setup, you'll need a stereo RCA to (whatever you need) cable. Most likely 1/8" jack if you are using the built in soundcard in your computer.

But to answer the original question.. a 12$ DI that doesn't sell in music stores...probably not a good choice

when you play somewhere, 9 times outta 10 they're gonna supply you with a di box. jussayin

All the connectors on the back are RCA? Can you show us the back?

it aint the best but I use it

Show us the mixer you have and I can give you a very specific recommendation.

okay I see, thank you. How many cables should I need if I have 3 gameboys, a 4 channel mixer and a kaoss pad?

You shouldn't need a DI before your mixer, you may need one after your mixer if the sound guy doesn't have one at a show. Worth carrying for emergencies like that if you have the room.

i don't think it would really do anything, making the signal balanced doesn't get rid of the hum from a gameboy.

@nick I was thinking of using it for my gameboys, then linking to a mixer. Does that sound like a bad idea? or contradicting

what do you need the di box for?