That question has as many answers as folks to ask it. Since YOU are the one asking and you have a very limited budget, the answer may very well be - as much as you can afford!
IMO that price range pretty much puts takes you out of the sub realm and puts you squarely into focusing your budget on getting the signal control (mixer and amp) questions solved before you start thinking about subwoofers.
For your uses, the PA should be more to support your vocals and get a good house balance, rather than try to inundate your audience with spine crunching bottom end.
For your budget, Golden Boy is right on track. The integrated mixer/amp is probably the way to go.
Try Carvin - They've got a really neat unit that retails under $600 that has 8 channels with full range contorls, 3 power amps (yep Right/Left/monitor), parametric and graphic EQ's, and a 24 bit digital effects unit built in. With this unit and a selection of 15" cabs with horns, you should have a rock steady, easily used PA with tons (tones?) of flexibility. IMO you could spend a lot more but you won't get anything more usable than this.
Hope this helps.
