cheap DIY nutrient dispenser?

I've seen these control boxes that act like a basic PLU and have these fancy dispensing systems all very over-engineered for the task. I'm thinking something more macguyver, more cheap Even if it's as complicated as a state machine that's something I can handle.

It seems like it'd take a lot of work, but it is possible. How about some sort of electric solenoid valve hooked up to a very specific digital timer. Set it up to where your nutrients will come out of the valve whenever you activate it and measure the amount that comes out over the duration of any given time. Then adjust from there to get your desired amount.

Although I would not normally recommend something like this because there is no room for error...One mistake can fuck it all up man. Just my 2 cents.