What would it cost to put a 30-second ad on the evening news?

It will depend on many factors such as what network the advertisement would be aired, local news or national, and the exact time of placement. There are agencies that can find airtime at a substantial discount. The way the discounts work is the non bought advertising spaces leave a network no choice but to allow a lower bid to fill in the advertising spot. There are some agencies that specialize in finding just that.