The pricing structure is very basic.
$2.50/month gets you a hosted Trilium Notes instance you can connect to from anywhere.
In addition, you can pay yearly with a plan that costs $26/yr (a 13% discount). This will be offered at checkout.
There is no hard storage limit, but if you plan on storing large media or files, this may not be the service for you. If you are storing significantly more than normal, in the gigabytes, and it's causing a reduction in performance or affecting other users, you will be asked to reduce your usage. It's not enforced or automatic, but a soft limit. Please play nice.
Trilium.cc will contribute 5% of your subscription to remove CO₂ from the atmosphere, in collaboration with the Stripe Climate program. They are very transparent about the projects they support, sending me quarterly updates on my contribution and which projects they were allocated to, so they have my support. You can find more information here.
To prevent the most catastrophic effects of climate change, we should aim to limit global average temperature increase to 1.5°C above pre-industrial levels, which corresponds to reducing global annual CO₂ emissions from about 40 gigatons per year as of 2018, to net zero by 2050.
To accomplish this, the world will likely need to both radically reduce the new emissions we put into the air, and remove carbon already in the atmosphere.