The
old pricing of Stackdriver that is valid until June 30, has a free Basic Tier. The
new pricing has free allotment per month. For logging: first 50 GB per project, additional GB cost $0.50 per GB. In Monitoring all GCP metrics are free. Pricing for Stackdriver Trace will be changed later as you can see in the table.
Basically for most projects (with GCP resources) the free allotments should be sufficient. The new pricing page also explains how you can use logs exclusion and exports to reduce logs usage and costs. It also includes a tool to estimate future costs (if any).