I usually go with the 2% of their body weight rule. I feed my two minis about 4lbs of hay (mostly grass) each day. I'll feed a little more in the winter or if they are working hard.
Those babies are adorable but with wicked bad feet. Probably fixable though if you are willing to put the time and/or money into it.
I paid $250 for an AMHA stallion under 32" with great blood lines. However, my friend paid $1200 for his AMHA mare. Totally depends on area and how bad someone wants to sell them.
Riding: The art of keeping a horse between you and the ground. ~Author Unknown