I put in a DIY system for my pond with 2 diffusers this summer, also about 12' deep and about .2 AC when full, for a bit over $600. The price for DIY can vary a bit depending on how much weighted tubing is needed and if you go with a quality distribution manifold. I put my system in a plastic storage container with forced air vents and it works just fine in all weather conditions.

As far as weighted tubing goes it's in the neighborhood of $1 per foot or more for 3/8 inch so it adds up fast. I found that 3/8 inch airline for compressors runs about 1/2 the cost and also sinks so I used that for distribution from the the compressor to the regular weighted tubing to cut the cost.

An off the shelf solution would be more costly but then you don't have to deal with the headaches of collecting & assembling all the bits and pieces. In my case I like doing that along with saving some cash.

All I had for over 10 years was a small fountain that ran 24/7 and we've still got the variety of fish the original owner stocked about 4 years before we bought the place - even with the pond completely freezing over during the winter months except for the hole from the fountain.