You're right, it can get pricey, especially after factoring in the price of equipment and labour. Not to mention that after you do add your amp, you'll likely still want to upgrade your speakers too for better sound quality.
Unfortunately this is just the way it is for us with base audio.
There have been people who have successfully upgraded their stereo using the DIY guides here, and have posted their success. It will take some time though.
Where are you located? If in the GTA I can recommend a good shop that did my stereo install and had reasonable rates and great quality work.