WPMUDev is a well know WordPress development shop with a strong history in multisite and plugin development. They built The Hub which allows you to manage multiple WordPress installs. Plus! The subscription gives you access to their other family of WordPress plugins.
Pricing starts a $19 a month for one site and goes up to $99 for unlimited sites. It’s an amazing deal considering their other plugins are some of the leading optimization and security plugins on the market.