It sounds like φτηνή to me. Having said that, φθηνή is an acceptable variant and should be marked as correct outside of the listening exercises.
That's good to know; I always make this mistake in listening exercises, though, so I suppose the only solution is to listen harder :-P
For some reason I always hear φθηνή instead of φτηνή
What's weird is that, in the regular-speed recording, it sounds clearly like a T to me, but in the "slow-mo" version, it's definitely TH....
Yes, that is odd since they are exactly the same (TTS). However, τ and θ are so close it's not surprising. In normal Greek conversation, you might hear either of them.
Try listening to the sentence a few times without reading it every time you get it as an exercise and eventually you'll get used to it.