We have 5KW PV systems on the roof at home and at work.
PV installation is such a competitive market that you really need to be careful of companies trying to pull the wool over your eyes to shave a few bucks of their costs.
The easiest scam to spot is the "only $1500 for a 5KW system" line. Read the fine print and you get a 5KW inverter and 4 x 250W panels. Similarly, avoid any deal with the word 'up-gradable' in it's tag line.
The next problem is a bit nastier. A few years ago, everyone's solar panels generated 180W each, then they were generating 200W each, now everyone is selling 250W panels... Whilst PV efficiencies have improved over the years, they haven't improved by 39%! A modern panel will produce 250W of electricity if a small star explodes immediately above it, and about 200W on every other day. Seriously though, we live in Perth Australia which has about the best weather you can get for PV solar. We see 5KW out of our systems for about 10 minutes, 3 days a year. Fortunately, there's a good fix for this, but I'll come to that in a moment.
The next problem is that the earth is irritatingly spherical. If you live at a latitude > 30 deg, then you are going to want to install the panels on an angle. If you are lucky, then you have enough angled roof space pointing in the right direction. If not, then you probably need to buy angled mounting frames, these aren't expensive, but they also aren't listed in the advertised price either. There is an alternative though, we have a flat roof at work and we didn't buy angled frames, I'll get to the magic fix soon.
The last thing to consider, is that panel outputs drop over their lifetime. You'll be down to 80% capacity after 6 years or so.
So... the magic fix.... Buy 10% more panels than your inverter will deal with. On a 5KW system, buy 22 panels instead of 20. At 32deg lat, it works out the same price as the tilting frames and delivers the same result. Alternately, you will get closer to the maximum power of the system on more days. I guess if you live further north or south, then you could add even more panels. It means you'll get more power for longer as the system ages, all of he installers we spoke to were happy to do this...
Oh, and price? In Australia, you pay around AU$1 per Watt of installed capacity at the moment. (might have come down in the year since we did ours)
These are just my experiences, I'm sure others will have theirs...