Hi Sam!
Firstly, please try to post in the correct forum ("Collecting Help").
Secondly, there's many topics about this in the "Collecting Help" forum,
this one on the first page which perfectly answers your question. {thumb2} In short, stamps from the country you're sending to is usually the cheaper option. If your SAE is bigger than 8x6", I recommend sending 2-3 IRC's. Anything smaller, and 1 IRC would do.
Good luck,
zanne