Driving ~3V hobby DC motors
If your motors draw less than 800mA each then an L9110S bridge driver should be sufficient. Boards with a pair of these drivers on them commonly sell on eBay for under $2 each (with free shipping, how do they do it?).   

However if your motors are the typical type used in toys (eg. Tamiya 980112M) then they may draw over 2A, which is too much for the L9110. In that case it may be better to make a bridge out of discrete transistors, preferably MOSFETS as they have built in flyback diodes and are generally more efficient. If you buy in bulk the price should be competitive, though the total build costs may not be (what is your time worth?).

So before you make a decision, get a sample of the motor you want to use and test it. If the stall current is less than 800mA at 3V (armature resistance of 4&ohm; or higher) then use the L9110S. If it is much higher then consider building discrete FET bridges or get less power hungry motors (maybe they cost a little more, but the total cost could be less!).