Smartphones are awesome fitness companions, but commercial armbands are expensive. If you're looking for an alternative and don't mind doing a little work, try creating your own out of a placemat.
Spanish website Las Manualidades shares this handy tutorial (via Google Translate). You'll need to cut a plastic placemat and glue it into a pocket shape to fit your phone, and then add some elastic hair ties to secure the phone into place and to keep the band attached to your arm. The finished product looks just about as good as a store-bought version, and you could even customise it using different kinds of placemats.
If the plastic doesn't breathe enough for your tastes, the basic shape and design is pretty universal, so you could substitute a cloth placemat, or even a bandana. Check out the source link for the full walkthrough if you want to try this yourself.