Does it cost money to send an iMessage to someone in another country?

No, you can send iMessages to *any iPhone, iPad or iPod Touch user world-wide for free.

*iPad, iPhone or iPod Touch must be using iOS 5 and above, and iOS 5 is supported on:

iPhone 3GS, iPhone 4, iPhone 4S, iPad, iPad 2, iPod Touch 3rd Generation and iPod Touch 4th Generation

I was going to say that too (: does also cost if your 3G and WiFi (not much though), and I should also mention @rachael you and your friend need an internet connection when sending and recieving an iMessage (3G or WiFi)

The imessge costs nothing howver you will need acces to WIFI (that costs !) otherwise it may default to a standard sms Message