I think it probably costs Google a little bit to queue up your email for a send action 30 seconds from now instead of sending it off immediately. I bet it would actually cost them money to just enable it for everyone - I'm sure the percentage of users who even explore Labs is very small.
I can't imagine the cost would be significant, relative to the overall cost of running GMail. And it's not like they generally try to run GMail on the cheap.
Perhaps they're concerned that seeing an 'undo' button would confuse people about how e-mail works, and they'd get a lot of requests to 'undo' messages that had really been sent. Or perhaps there are enough cases where people really want the message to go instantly that even 30 seconds delay would annoy them.
This actually started as turning a bug into a feature.
The gmail servers delayed sending the email for about 5 seconds normally, all they did was add a cancel button.
