I might have considered around $5/month - maybe even 10. But all I really wanted was Ring integration, location services, and webhooks. Just a little glue to tie a couple things together, and particularly do the location-based triggers.
But Stringify, for me, has been badly broken for the last few months at least in terms of reliable location triggers... so that monthly payment would have to see some real returns on seeing the service perform to the level I needed. And maybe $5/mo couldn't buy that.
Any rate, I don't use SmartThings, and Yonomi doesn't support what I need. IFTTT has traditionally had reliability problems and very slow responses. Few really good choices left.