Not sure what you already have, but Brachypelmas are very cool, but still cost a good few bucks.
If you want something really cheap that will develop awesome colours, go for a Nhandu Chromatus (Brazilian Red & White), you can find them as cheap as R30 or R40 and they develop great colours when they grow bigger!
Also, Poecilotheria Fasciata (Sri Lankan Ornamental) an Old World Arboreal with great patterns on it's body and is very affordable and exciting to keep. Has a nasty venom though so be warned.
Hmmm, you can also get Cyclosternum Fasciatum (Tiger Rump) not very expensive (R100 to R150), but eats like a monster and grows quite well. Shows great colours from young already.
My 2 cents.
HS