Sure if you compare the entry level Mac vs a PC using an expensive micro ATX form factor then the difference isn't that big but if you drop the micro ATX you can come in a fair bit cheaper.
ZA Store :
20-inch widescreen LCD 1680x1050 resolution
2.16GHz Intel Core 2 Duo processor1
2GB memory
250GB Serial ATA hard drive2
8x double-layer SuperDrive (DVD+R DL, DVD±RW, CD-RW)
ATI Radeon X1600 graphics with 128MB GDDR3 memory
Built-in AirPort Extreme and Bluetooth 2.0
Apple Remote
R19349.00
PC Palace :
204Bt 20" WIDE TFT, 1600 x 1050, 5 Millisecond R2,382
Core 2 Duo E6600 2.4GHZ, 4MB L2 Cache, 1066Mhz Frontside Bus from prophecy.co.za R2673.93
Intel 'Westchester' G965 mobo R1,053
Gigabyte X1 Chassis - Black 400W PSU R423
2GB DDR800 memory R2,083
250GB Serial ATA hard drive R649
LG Super Multi DVD Writer GSA-H42L Internal IDE (Dual Layer) Light Scribe
(DVD+ R 18x, DVD-R 8x, DVD+RW 4x, DVD-RW 4x) R316
RADEON X1650PRO 256MB GDDR3, Dual DVI ( Dual link ) R997.5 from prophecy.co.za
R 11454.43 excl
R 13058.00 incl
There you have a R6300 price difference excluding the Apple Remote and Airport stuff and it has a better CPU, better gfx card and a more powerful PSU.
Even if you switch to micro ATX with a fancy case it'll still be a fair bit cheaper.