A fairly economical servo ($32.99) that I have used with great success on my Boomerang XL is the Futaba S3305. I use them on all of my flight surfaces. These work so well that I am beginning to question the need for $100+ servos. Check them out here:

http://www.servocity.com/html/s3305_...l_gear_bb.html

I am currently equipping a Grumman Panther with them.