Slipe pretty much answers your question but here is another way of looking at it...
Well, let's try to calculate the megapixel requirement (someone correct me if I'm wrong):
A4 paper size = 210mm x 297 mm ~= 8.27 inches x 11.69"
If you need 300dpi, that is a resolution requirement of roughly 2480x3508.
total resolution=2480*3508 ~= 8,700,632
So you need an 8.7 megapixel camera.
I could be wrong but I don't think there are any 8.7 MP prosumer cameras so affordability is going to be a question. I'm not sure about DSLRs though.