I love Blink, and it is open-source.
But this is one of the apps I cannot believe is not subscription-based. Author keeps up with changes in iOS and improves constantly.
The cost is justified too as the market for terminal emulators is much smaller than general iOS audience.
I don't do any professional development on my iPad, I just like having the ability to mosh/ssh back into my machines at home to run a script or push a quick edit to my personal github.