It depends on how much power you are consuming. The investment varies.. try looking through this link:
Solar Photovoltaic Projects
Here is a detailed guide on purchasing PV panels:
http://www.homepower.com/view/?file=HP121_pg70_Schwartz
[EDIT]
I'm thinking we should start a CE project that ties with renewable energy.
I'm planning to install a mini Solar Power system in my family house for uni research purposes, but I certainly don't mind to getting involved with a scalable CE Solar Power project
By the way, Koli, one estimate of the costs is $3 per watt. P.V panels sizes differ from one manufacturer to another, even with the same ratings, so I think the only comparable measure is dollars per watt, which ties in with the conversion efficiencies too.
Oh, one has to remember that these are one-off costs. Once purchased, you'll have a long lasting electrical generation for free. Some panels are warranted up to two decades I believe. Also, its better to progressively install P.V systems. Over time, the price of panels will decrease allowing for cheap upgrading!