Autoscaling Heroku workers is definitely a problem that people would like solved, but I see a few issues with trying to offer the solution as a SaaS. One, you're targeting developers. There are several github repos that do this kind of thing for free, and developers would know how to find and implement those. Two, those github repo's are going to do a better job of scaling because they can turn dynos/workers on and off instantly (e.g. as soon as a delayed job is done); much more cost effective than waiting up to 5 minutes to turn dynos/workers on/off. Three, you're targeting people that are trying to save money and _don't_ want to spend money.

As an aside, perhaps I (a cheap developer) am not in your target demographic, and you're really targeting folks with lots of Heroku apps and lots of money to spend. In which case you should be charging bundles more than $20/month.