The cost will also depend upon your phone service provider.
If you have a reasonable 'inclusive minutes' tarriff, sending and receiving plain text e-mails in the UK isn't too bad - but on a non-GPRS service at 9.6Kbps, accessing the Internet is painfully slow and can be expensive with the high graphic content of many home pages these days.
I use a plug-in Vodafone Mobile Connect GPRS card - but even that is pretty slow and NEVER reaches the advertised top rate of 57.6Kbps. Using it overseas is quite expensive, so I only use it as a last resort if I can't find a local WiFi hotspot.
3G? Very expensive at the moment and has poor coverage.