The cost of usage is strange enough that it warrants a separate point in this point list.
Cost of acquisition is no contest; even the most expensive Internet Mail product is cheap compared to X.400 products.
Cost of platform: Most Internet Mail products have a much smaller footprint in memory and disk usage than X.400 products. (Sendmail, not the simplest product in the world, has approximately 30.000 lines of code, and a single 160 Kbyte executable under Linux.)
Cost of installation: This is largely product and configuration
dependent; the Sendmail config file I use is 20 lines long, others
have good reason to complain about its labyrinthine configuration.
DNS routing means that an area which can get quite complex in X.400 products is simply not a problem with Internet mail; on the other hand, many X.400 users simply configure a connection to their service provider and let the service provider handle "all that".
Cost of training: Again product dependent. The text format of Internet mail messages seems to have encouraged a larger commonality of interface (you should be able to get at the header text somehow), leading to greater transfer value of knowledge between products; the "ugly address" problem of X.400 has tended to go in the other direction, since many vendors have their own style of address hiding in their user interface.
Cost of time in use: Totally product dependent. People reared on Unix
systems may complain that it's impossible to use "grep" to find
messages in their ASN.1 encoded X.400 mailbox, but they are a
Some people argue that the binary, strictly defined syntax of X.400 makes more powerful functions possible; I don't know if this has proven true in practice.
Cost of support: Product dependent.
Cost of maintenance: Internet mail products tend to be well integrated
with UNIX user bases, and the DNS routing factors out the routing
Internet mail error messages are often nearly inscrutable pieces of pseudo-English; X.400 error messages are standardized to the point of incomprehensibility. Both approaches have problems.
Cost of usage is covered in a separate point.