Polomon.com is in a campaign and offers the MS Visio Pro 2016 for $17.99 (USD). A couple other stuff are in campaign as well, MS Project for $17.99 (USD), MS Office for $15.99 (USD), and Windows 10 Pro for only $9.99 (USD).
This is an US site, but the keys works well all over the world.
I don't trust polomon.com at all. The site is only 3 weeks old.
Their contact email is at gmail rather than their own domain.
The prices are too low to be genuine. Your keys could be disabled by Microsoft at a later date.