Cheapest audio solution

What's your budget? Do you have the JBL sound system? The following applies for the base system.
For $100-300 you can get a decent headunit ($100-200 for single DIN, $200-300 for double DIN to fill the space). Decent speakers will be about $200 for all 4 doors. Miscellaneous stuff like the harness and other supplies should be under $50. Subwoofer would be $100+ depending on what you want. Subwoofer is optional, but I like the added bass of even a small unit.
The headunit is easy in the Prius. I haven't done the speakers, though, so I can't comment on them.

 

If you have a non-JBL (i.e., base) system, you can install a stealth single-DIN unit with standard tools, 1-2 harnesses and a few hours of your time. You can purchase a single-DIN unit with a disc player (CD/MP3), AUX input and USB port for under $100 if you look around. You can do the same with a JBL system, but it's more costly and involved. How much improvement you'll get without replacing speakers is another issue. Most people report very poor fidelity with the JBL amplifier and stock speakers.

If you have a JBL system, you will either need to purchase a new amplifier or an adapter which allows an aftermarket radio to be used in conjunction with the stock amplifier such as the Metra TYTO-01 JBL Amplifier Interface ($52 from Amazon). If you choose the latter option, the aftermarket radio you choose will also need to have RCA line level outputs.

I went for the JBL interface and a single-DIN for my '05 Prius. The sound is not great, but better than the original head unit. However, I'm far from an audiophile. I did my installation mainly to have mp3/USB/AUX input capability, as my head only only played standard audio CDs and had no ports/inputs.

Check out the Stealth install an aftermarket stereo thread if you have not done so already.

 

might go to crutchfield for some better speakers; they usually hook you up with install instructions. if you have a friend with some common hand tools (and sense) you should be able to upgrade your speakers pretty easily. only problem is the jbl speakers are like 3 ohms so you need to ask them which speakers have the right resistance. they should be able to hook you up.

also, I use "duct insulation" from home depot to replace the plastic sheet vapor barrier in the doors. not as good as dynamat, but much cheaper and lighter and it does work good. its a roll of 1/8" thick foam with foil on one side and glue on the other.

alpine tweeters clip right into the front door triangles, not sure if they fit the rear doors cause I don't have jbl.