I'm using Pondmaster AP100 pumps which can be had for around 200 or less if found on sale. It is a pump on the same type as the Eco 7 mentioned above. Add a diffuser and sinking hose. The hose will be the expensive part of the project if you have far to run. You can run inexpensive black plastic water pipe to waters edge and then the sinking to the diffuser.
The linear motor pumps are good for about 8 or 9' maximum depth. Their air flow drops off rapidly with depth so they are only suitable for shallow ponds. I'm running mine at 8-9' and that is as deep as I would recommend, although I think advertised max depth is 11'. At 11' it would be putting out hardly any air. I get a good water boil at 8-9'.
Kits are available for diaphragm repair on the Pondmaster pumps for anywhere from $8 to 20 depending on where you get them. The shipping is often almost as much as the kit.
Here is information on my installation. Later in the thread I think there are pictures of a housing I made for the pumps.
Snrub areation system More pictures including pump housing For a single pump a portable dog kennel would work.
Home made diffuser base This post and the next post below it pictures of a home made diffuser base
diffuser base modifications Some modifications I made to dual diffuser bases and some things I learned along the way
I'm not saying this is the best system in the world. In fact I will say it is NOT the best system in the world. But it does work. The pumps seem to last for about 10-15 months running 24-7 before needing a diaphragm rebuild kit. Some day I likely will buy a "good" pump and run the three double diffusers off of one vane type pump. Will likely keep running the sediment and forage ponds off one of these pumps. The pumps work fine for shallow water and are one of the most electrically efficient type pumps (for low pressures = shallow water) but they are not the longest life pumps. They do need periodic rebuilds. Takes me about an hour to do the first one and 30 minutes or less once I got the hang of it. Someone competent could probably do it in 10 minutes. Not rocket science but do have to pay attention to how things come apart and go back together.
It is a way to experiment with a cheap system. Everything under water I used top quality stuff so all I would have to do is upgrade pumps if I wanted a better system. In the beginning was not sure if I even wanted aeration so this was a way to experiment without getting too much money invested in the first pump and diffuser. Not the best way to do it, but the way I did it.