In the typical case such a device is really only useful in the first month or two. After that all optimizations have been done. So I think that this monthly subscription model is a bit flawed in that regard. I'm probably wrong..
There are a lot of measures that can reduce people's power usage for a short period of time. For instance, advertisements to ask people to turn off unused lights can have a temporary effect on power usage. Making those savings last is much more difficult. Hopefully ongoing power monitoring can help there.
For Gridspy, I'd love to provide a free minimal service, a highly affordable normal service and a premium service with long range high resolution data.