What I need is a similar thing to what is happening at:

http://www.d2deals.com/east/barmor.shtml

When the quantity is changed, the price changes on the place where it is visible to the user, and in the hidden form input. The levels change accordingly:

amt
1 = base price
2 = 10% off / 90% of base price
5 = 20% off / 80% of base price
10 = 30% off / 70% of base price

Also, the script rounds the price off to the closes $n.50 or $n.99

Could someone at least point me in the right direction for this? Or even an entire script would be appreciated.