Go buy some cheap perstaltic pumps online. I don't have a link handy, but if memory serves me right theyre around $30. A cheap mechanical light timer will allow you to automate your doses. Use a cheap rubbermaid tub for a reservoir.
These pumps generally have a flow rate of something like 10-20mL/minute, but it depends on pump RPM and tubing size.
I use my autodoser as an auto top off also. I lose about 1/2 gallon/day to evaporation in my main tank, so I run my pump long enough to pump in 1/2 gallon each day (about 4 hours in my case), add 3 1/2 gallons of water to the reservoir, and insert a week worth of macro nutrients.
I only run this system while I am away for more than 3 days at a time which is pretty common due to my current job. I just dose micros before I go, and maybe a double dose if I'm to be gone for a week or more.
I have my brother in law watch my apartment while I'm not there and this system keeps him from having to fertilize, or do anything besides throw food in the tanks. (He is not an aquarium enthusiast.).
All in all, it will cost you about $35-40 to put this system together, $70-80 if you set up a microdoser also.
Edit: Pretty sure I bought mine here http://www.aptinstruments.com/Mercha...ory_Code=SP100
. If so, they're about twice the price I quoted. They may have gone up in price or I may be senile. Both are about equally likely.