What is expensive to you? For me, I think $50 for a set of speakers would be cheap, and that's the Alpine range. This is one of those things where you get what you pay for -- if you spend $20 on a pair of speakers, don't expect them to be very good. Also, I wouldn't buy Sony, but my friend has them and they sound pretty decent for what they are, though I'd still recommend Alpine over them.
P.S. Don't try to make a frankensystem out of some of your Bose stuff (this means the amp too) because it's all proprietary and tuned for the stock setup afaik.