After experimenting with my Motorola C139, I have determined that it costs 0.3 units to send a text message and 0.3 units to open a new text message. You can also delete unopened text message for free.
My wife's Nokia 5100 charges 0.5 units to send a text message. I have not been able to receive a text message so far so I don't know how much it costs.
Google has a few neat SMS services. The first one is Google SMS. This works with my phone but I never received an answer on my wife's phone. The next one is Google Send to Phone. One problem I encountered is that you can't bookmark the site, when you try to use the bookmark google throws up. You need to use the above link. For my C139 I need to use the Cingular network to receive text messages.
I tried sending my C139 a message using Teleflip.com. You just send an email to firstname.lastname@example.org. I tried several times, but I never received the text message that I sent. The email bounced back to my inbox either.
Another service that did work was Jason Holm's site. The message appears from email@example.com. I'm not really comfortable with using somebody else's personal email address to send myself text messages. Looking at the source on his page, I can't figure out how he does it. The web page just sends a standard POST form to him. The magic must appear somewhere else.
Knowing that Google sends text messages to my phone via the cingular network I tried using firstname.lastname@example.org. The text message was bounced back to my email box. It said the reason was: User not local and forwarding not allowed.
Here is a link for some nifty things you can do with SMS.