So, one $30-50k engineer gets split 5-10 ways on infrastructure. Most maintenance is automated with setup and occasionally fixing something using up most time. You usually negotiate 1 day to a week of dedicated time out of the month w/ split being flexible.
Heroku and Redis might still come out a better deal. It's just not going to be nearly as different as people think because one doesn't have to load-up on IT people to deploy or maintain common configurations. All kinds of consultants and smaller fish that will do it way cheaper [than you mentioned] with cost spread across multiple, small businesses.