It depends largely on what army you want to play, a horde army will set you back a fair amount but you\'ll have tons of models and a small elite force will be more expensive per mini but cheaper in the long run. Space Marines are probably the best starting point for beginners, you don\'t need too many and they\'re pretty forgiving for new players. If you\'re pretty much sorted for paints etc. that\'s a huge chunk you won\'t need to spend.
The Assault on Black Reach box is going for $75 from GW (but you can get it much cheaper from other sources) and contains the basis of a Marine army (and some Orks to mess about with too

). One unit of Marines, a Commander, a dreadnought and a unit of terminators. That comes to about 600-700 points so grab a box of scouts to go with the two guys you already have and you\'ll be set with a 750 point army which is ideal for learning how to play.
The rules come with AoBR but you will still need a Marine Codex, those go for $30. So all added together that makes:
AoBR - $75
Codex - $30
Scouts - $20
= $125
If you want to expand up to 1500 points which is a normal sized game I\'d recommend the SM battleforce for $90 and a Command Squad for $35. The battleforce would add another Tac Squad in a rhino, another 5 scouts to give you 2 units of 6 in total and an assault squad. I\'d leave this last bit until you find out if you actually like playing the game or not.
I hoped that answered your question and that you have fun trying out 40K.
Edit: looks like several people beat me to it but it looks like we all have pretty much the same advice. I don\'t know where you guys have been playing but round here the standard sizes for games have always been 1500 points for 40k and 1500-2000 for WFB.