The non-profit scientific research organisation Project Drawdown notes that implementing a certain category of agroforestry called “multistrata” – a combination of long-lasting crops and multiple layers of trees or vegetation – in humid tropical climates would cost more than $1,300 per hectare.

Given suitable conditions, the plants will grow. You don't need to pay them. There is no need to complicate it. Establishing a productive agroforest only costs the time and effort of actually doing it. Spending money can make some aspects more convenient, such as buying seedlings from a nursery rather than going out and collecting seeds oneself, but surely if the motivation is there, that labour can be done by volunteers. Let's not scare people away by telling them that it's expensive to plant trees. The more people doing it, the easier it gets for everyone.