if you have to get an aluminum head on a pontiac fixed?

Normally when these engines are over heated its not pretty, and can cost upwards to over 1000.00 in p[arts and labor. And normally there was a cause which created this state of overheated condition.
1/ water leaks
2/ fan troubles
3/ electrical troubles
4/ radiator troubles
5/ thermostat/ over fan relay or switch malfunction, and or wiring
Check to see what the car is worth before repairing, that this car is blue booked very low for a major repair.

Good Luck

100 dollars?

Depends on what is wrong with it.

Without more details, it would be hard to guess.
Chad's guess of $100 would cover the gasket kit that's needed everytime you remove the cylinder head. What it would not cover is any other replacement parts, supplies or millwork that might need to be performed on the cylinder head. So add perhaps another $500. Now lets talk about the labor charge to do all that work. Figure at least $75 per hour. I would say the removal and replacement of the cylinder head would take at least 4 hours.
So a rough guess would be about $1000 to do the job.

Depends on what's wrong with it.

Regardless, it will be more than $100 like the other poster mentioned. If it's warped, it's totally gone, you need to replace it completely.

if it just needs valves, or something of that nature, around $500+

Edit: If it's cracked, it's gone. You can't mend a cracked aluminum head. Say hello to a VERY costly repair.