I've seen reliable solar aerators. Inexpensive and simple - NO. But they were the only ones I've seen. They were the industrial/commercial type. At one time - I considered this for my ten acre lake.
Cost and maintenance were the reasons I didn't purchase one or a system.
I'm with @oosik; I know of cheap ones, and I know of reliable ones, but there isn't any overlap that I know of.
What is your goal with the aeration?
Is it a big pond? A shallow pond? Do you have excess water? Fish? Algae blooms?
For cheap, I would think of a vertical windmill (55 gal. Drum cut in half) on a shaft turning an impeller wheel at the surface, but check your maximum wind speeds, and think about a clutch or a brake.
Do you want a bubble system or do you want a fountain system?
I would think that a bubble system would be far easier and cheaper to install and maintain than a fountain system. But then you might just want the fountain cause they are pretty to look at. For a bubble system, making your own would be fairly easy. Since its outdoors, you don't need UL listed solar panels, and these used, can be as cheap as $50 for a 100 or so watt/ 12v panel. At least in my immediate area, these are available locally. The air pumps look to be in 30 to 50 dollar range. I would get a 24 volt air pump and set up two, 100 watt, 12v solar panels in series. And you can make your own bubbler tubes. I would check what the the amount of noise the pump makes before purchase. It should be quiet. A system like this doesn't need any control switching, its always on. And I would not care how the panels are orientated, as efficiency isn't that important.
Fountain systems can be a pain to maintain, as you will have to filter water to the water pump somehow, and nature WILL find a way to mess up that filter. A bubble system doesn't have this issue.
Back when I was a boy scout, I stumbled upon a homemade pontoon boat about 4' x 8' floating in the center of a small pond. The boat had a 6' tall windmill with a shaft driving a propeller just under the surface. Both ends of the boat were tied off with a cable running from shore to shore to keep it centered.
That was the coolest collection of scrap materials I had ever seen.