When you look at bang for you buck, Its hard to go past second solar panels, These were hard to find years ago and not cheap when you found some, Supply and demand.
Now that's different, due to people upgrading their solar systems. For whatever reason. maybe their inverter died as so many have, and they got talked into buying some new panels too by a fast talking salesman.
The outcome is good for us and there are plenty of cheap second hand panels to be had, just ask your local solar installer if they can help you find some, or look online.
Most second hand 24 volt panels (72 cell) are 2008 or newer and are claimed to last 20-25 years so should have a good 10 years left in them, I would trust the name brand ones over others and pay accordingly.
I Always take a multi meter and check them all, You can test them under load with a couple of 12v head light bulbs in series if its sunny.
Current prices vary, I have seen used panels over 200 watts fetching around $0.50 a watt E.G. 200 Watt = $100 and 250 watt panels around $125.
and for the bargains, panels for as little as $0.30 a watt, now I think that's a deal. I saw 2.15 Kw of 215 watt panels for $720.
And for Absolute bargains look for Thin Film panels or Amorphous just watch their output voltage is higher and check they suit your application but with MPPT solar charge controllers. On the down side they have about half the watt output size for size as the silicon cell panels, but if you have plenty of roof who cares.
After years of dicking around with this stuff,
I try and buy new batteries, new charge controllers and new inverters and cabling and fuses etc and Used solar Panels, You should be able to go to the corner shop and buy an ice cream with your savings, that's if you can find a corner shop anymore, Hi.