I spent £30 on my entry level stuff, i.e.£30 on set of components, another £30 on coaxials, cos to be honest the Zed does create a fair bit of road noise from the rear due to the open boot area.
Unless you soundproof all of that, there is no real need for premium equipment.
Although i did spent decent money on a decent amp and ripped all the Bose/Clarion stuff out