OK ... one day only ... this day!
The core Tone Stack App is .99c but the real deal is on the "Mother Load" which is going for $13.99 as an IAP. So that's $14.99 for everything.
I've been told the effects in Tone Stack are amazing by a guitar tone connoisseur no less. They can also run in an AudioBus effect slot, so you could use them with another Amp sim like Bias if you felt that need.