Audiobus: Use your music apps together.
What is Audiobus? — Audiobus is an award-winning music app for iPhone and iPad which lets you use your other music apps together. Chain effects on your favourite synth, run the output of apps or Audio Units into an app like GarageBand or Loopy, or select a different audio interface output for each app. Route MIDI between apps — drive a synth from a MIDI sequencer, or add an arpeggiator to your MIDI keyboard — or sync with your external MIDI gear. And control your entire setup from a MIDI controller.
Download on the App StoreAudiobus is the app that makes the rest of your setup better.
Comments
You might want to find a TC Helicon Blender with 6 stereo inputs, it doubles as an iPad compatible audio interface (via Apple's Camera Connection Kit) and has pan and volume controls.
A larger, classic one is the Behringer Xenyx 1202SFX.
Both are below $200.
You are looking for strictly a summing mixer? Or do you need level controls? If you’re just looking for a passive summing mixer, diy recording equipment sells an 8 and 16 channel model kit for about $50, super easy to build.
The little Moukey mixers might be something for you. They are USB powered. I am using them as sub mixers, or because of the USB power in a portable setup:
Moukey MAMX3