buy a fountain pump or water pump... get some plastic transparent or that black film base tubing that fits the pump.. get a couple y valves and add on till you can have a tube for each plant.. tape them to a pencil that you can stick into the ground/soil/growcube/... get a bucket for the pump and fill the bucket with water and place the pump into the bucket with everything connected.. turn it on and it'll do its job..

if you want it to water automatically, get a timer socket as well..

total i'd say u'd b payin about $42

$20 for the pump, maybe 15
$10 for the tubing
$7 for the valves
$5 for the timer..