That's what I thought. I don't usually buy points but -30 jump seemed high. I'll probably just roll with the -3.5. I checked Scoresandodds earlier and it went from -3.5 to -4.5 quickly so I was surprised that it was -3.5 on my book.
I just looked and the price to buy down the Falcons from -7 to -6.5 and the juice was -130. The juice for Seahawks from -3 to -2.5 is -140. Conversely, the juice to buy the Steelers from +6.5 to +7 is -130. So its definitely not only when you buy down.