There are several good choices in the 600 to 700 dollar range, but not so much in the 500 range. If I were on a shoe-string budget but still wanted strong ozone I would buy two ASO 250's from AquaSun and an oxygen regulator. I would then lease or buy an oxygen tank to make good quality ozone. Then, with tubing, I would take the output of the first ASO 250 into the input of the second, thereby greatly increasing the ozone output. You see, even real strong ozone is only 15% or so of the total of the gases coming out of an ozonator when pure oxygen is used as feed gas. The other 82 to 84% is oxygen. The second ASO 250 increases the ratio of oxygen to ozone further.
Something to think about......