@christ.roux72, Let's Encrypt never plans to charge money for certificates; they should always be available free of charge.
As @svennd mentioned, the certificates are currently only valid for 90 days. That means you have to apply for a new certificate after 90 days. But you don't have to pay for it, you just have to get a new updated certificate, which is still free of charge. The need to reissue certificates periodically is not a way of making people pay money, but is intended to limit the damage if an incorrect certificate is issued at some point.
We're working on technology to make the automatic renewal and reissuance of certificates happen without human intervention. There are also other people who are working on scripts and tools that could help with that in various software environments. So the eventual goal is that your certificates will have to be reissued regularly when they expire, but that this will happen without any human intervention or effort, and without any monetary payment. It should eventually be very easy and automatic!
Let's Encrypt is run by Internet Security Research Group, a not-for-profit organization that is funded by donations (so far mostly from large technology firms, but also from other kinds of organizations) rather than by charging for its certification services. An earlier thread on our forum about this was