It's free if you just want to reserve a username and add an RSS feed from your own website - which can be either full articles (which will appear in Micro.Blog as a title & link), or short 280 char posts. You can reply to posts with a free account too.
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EDIT: Probably easier if for the other plans I just post the site's own info:
New Microblog — $5/month
We'll create and host a microblog for you at username.micro.blog or your own domain name. Includes cross-posting, pages, themes, and publishing from the web, iOS, and Mac.
New Blog + Microcast — $10/month
All the features of a hosted microblog plus audio hosting. Upload MP3s via the web or use the companion iPhone app Wavelength to record and edit your own microcast. We'll create a podcast feed for your site.
Enable Cross-posting — $2/month
Already have your own microblog? Add Twitter, Medium, LinkedIn, and Facebook cross-posting via Micro.blog. Works with any RSS feed.
Give Micro.blog
Invite someone to Micro.blog or pay for their first year of blog hosting.
Nope, the $2 is just if you also want it to cross post your RSS out to Twitter, Medium et al. If you only want it to go into Micro.Blog, that's free. The pricing page makes it a bit clearer:
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EDIT: Probably easier if for the other plans I just post the site's own info:
New Microblog — $5/month
We'll create and host a microblog for you at username.micro.blog or your own domain name. Includes cross-posting, pages, themes, and publishing from the web, iOS, and Mac.
New Blog + Microcast — $10/month
All the features of a hosted microblog plus audio hosting. Upload MP3s via the web or use the companion iPhone app Wavelength to record and edit your own microcast. We'll create a podcast feed for your site.
Enable Cross-posting — $2/month
Already have your own microblog? Add Twitter, Medium, LinkedIn, and Facebook cross-posting via Micro.blog. Works with any RSS feed.
Give Micro.blog
Invite someone to Micro.blog or pay for their first year of blog hosting.