Yeah, dropping $500 a month for 100 accounts can feel pretty steep, especially since that doesn't even cover the cost of getting the IG accounts themselves. If you're looking for alternatives, you might want to check out https://jarvee.com/ which is a powerful automation tool supporting multiple platforms, great for tasks like auto-follow and scheduled posting.
https://followadder.com/ offers targeted features like auto-liking and DMs. And http://rootjazz.com/instadub is excellent for large-scale campaigns, with mass account management capabilities. Just remember, all these tools come with risks, as Instagram's policies don’t permit automation, so use them cautiously to avoid account issues!
