miniSynth Pro from Yonac is on sale now for $3!
This is the biggest sale ever in the app's 2 year history. Despite its age it has been kept modern with updates that include Virtual MIDI In as well as background audio. I've never played with it though, so I can't speak to its quality.
Here is a little tutorial from Yonac on recording and exporting. It is all fairly obvious, but it is a good chance to hear the synth.