The cost depends on the resourcefulness of the buyer. I've seen a 1000 watt setup that was built for $5000. Home systems are based heavily on the storage system, the actual wattage used coming from the batteries, not the panels themselves. If a person is inclined to set up solar, you can go all in at once or just a little at a time, honestly.
I'm considering going back to school and solar is one of the avenues I am looking at. This subject is completely fascinating to me...
Gzapata, I saw an article a couple of years back about a team that was looking to extract energy from background radiation. including cosmic rays, but haven't heard anything since.
As far as collisions go: most of the space junk is concentrated in a couple of bands at specific altitudes so if you can avoid them you're okay. IIRC, geosynchronous orbit is a lot less dangerous than low Earth orbit.
But collisions is one idea why I like the modular idea - lots of small solar sats transmitting power back to a single transmitter for relay to Earth. That way too, you don't need big new boosters to get the powersats into orbit.