There's also a place in Phoenix that's selling surplus residential panels at even lower prices:

https://santansolar.com/productsAt first glance it looks like a great deal, except you'll have to buy a seperate MPPT controller to bring their outputs down to 12 volts, or give up 1/2 or more of their rated power.

You can mix 30 volt panels with 18 volt (12 volt) panels in an existing system but the higher voltage panels will de-rate themselves unless they have their own MPPT controller. The panels won't be harmed, but paralleling mixed voltage panels will drag the higher voltage ones down to the lower voltage, reducing their power output.

250 watts at 30 volts is 8.3 amps. If your system is operating at a nominal 14 volts output, 14 volts x 8.3 amps = 116 watts. That's what your 250 watt panel will put out if it's placed into a 12 volt system without a MPPT

And the MPPT has to be controlling only the higher voltage panels by themselves. Otherwise you'll run into the same problem at 18 -20 volts instead of 14 volts.