Highly likely that it can and is being done (please note that caveat at the end of the post), the NSA has already done this before (we are talking pre-PRISM era). The operation was named Room 641A. Where they eavesdropped all traffic. Consider that AT&T is a tier 1 provider and thus operates a large part of the internet.
Another instance which could be probable is abusing the backbone protocol of the internet BGP (Border Gateway Protocol). The protocol is meant to perform policy based routing before taking cost into account. This can, for instance, impede traffic from certain geographical area's reaching another Autonomous System. In practice it has been abused several times to re-route parts of the internet (if you actually manage to do this it's highly likely you are interested in the passing traffic). Refer to this article on Arstechnica.
"This year, that potential has become reality," Renesys researcher Jim
Cowie wrote. "We have actually observed live man-in-the-middle (MitM)
hijacks on more than 60 days so far this year. About 1,500 individual
IP blocks have been hijacked, in events lasting from minutes to days,
by attackers working from various countries."
At least one unidentified voice-over-IP provider has also been
targeted. In all, data destined for 150 cities have been intercepted.
The attacks are serious because they affect the Internet equivalents
of a US interstate that can carry data for hundreds of thousands or
even millions of people. And unlike the typical BGP glitches that
arise from time to time, the attacks observed by Renesys provide few
outward signs to users that anything is amiss.
"The recipient, perhaps sitting at home in a pleasant Virginia suburb
drinking his morning coffee, has no idea that someone in Minsk has the
ability to watch him surf the Web," Cowie wrote. "Even if he ran his
own traceroute to verify connectivity to the world, the paths he'd see
would be the usual ones. The reverse path, carrying content back to
him from all over the world, has been invisibly tampered with."
Nobody being capable of re-routing parts of the internet is interested in what you write in an email. Most of the time they are interested in specific targets. So if you are to make a formula, it would depend on the complexity of setup and the estimated information value.