Even if it's a bulk mail rate, and even if that rate is set by the government, this cost is enough to make it uneconomical to spam the U.S. Postal system. As a result, junk mail seldom rises above a minor inconvenience, a few too many catalogs from Pottery Barn. Not so with email. Internet providers customarily charge a flat rate for a server that can be used to send email, and there's no postage required on email. Moreover, the recipients often bear significant costs.
Some people already pay per message received or per hard drive space consumed on a mail server.
It's possible to take action on your own against spam. You could examine the message, trace its origin, and report the spam to the administrators of the network from where it originated. You could purchase anti-spam software or spend time configuring your computer to rely on the free, though not particularly effective, MAPS Realtime Blackhole List. If the message appears fraudulent, you could contact the Federal Trade Commission or state government offices. To an economist, all of this sounds familiar.