With all due respect to Gary, I would not play a keyboard through a guitar amplifier. The guitar amps are optimized for the frequency range of about 600 hz to 6 khz, whereas a keyboard, especially an arranger, produces a full range signal, with bass and drum lines, which could go as low as 40-50 Hz, and violins and many other instruments going above 16 KHz. The guitar amp, which usually has a single speaker, will usually not reproduce the arranger sounds correctly. The keyboard amps and bass amps are both optimized for low frequency response much better than a guitar amp is. However, the bass amps usually do not have good high-end response. The moral - try to use a keyboard amplifier.
To do it on the cheap, use computer speakers. The Yamaha YST-MS50 is an excellent choice. I bought mine for $85 at CompUSA, and they are great. You should also look in the computer stores or on the internet - they will be much cheaper than at the music stores.
The other, slightly more expensive option -
http://www.musiciansfriend.com/srs7/sid=.../base_id/100216 This is Rogue 1560 "stereo" amp at Musician's Friend. It is supposed to have stereo horns along with a single woofer, and is rated for 100 Watts. This might work.
In fact, I would much prefer to use a stereo sound, to take advantage of the the stereo samples played by your keyboard.
Regards