Here is the problem. Without spending a good deal more then $80, it is my opinion that you won't be able to get anything that is reliable. It's not to say it won't work, or that it won't last a long time without failure, that just means under $80 you might have a big failure at some point.
Thius is why I still dose excel. I don't like it, but I'm not comfortable with putting my fish at risk going the cheap route. When I do go with CO2, I plan out getting a dual stage regulator, from a recognized brand, that is meant for low pressure. I think i may even run 2 solenoids, incase one fails. and all the rest high quality parts. Last time I looked through things I was looking at 300-400 bucks. So for now I just deal with my excel dosing.