LoadForge launches it's load generation servers on your DigitalOcean account. You then incur a fee for running the droplets for a few minutes. Typically, this is several US cents to a dollar.
LoadForge always provides a cost estimate before launching a test for you. However, to understand the "costs" we are provisioning $40/month VMs in most cases, in tests over 50,000 users $80/month and over 100,000 users $120/month.
DigitalOcean does not yet support per-second billing and so you are charged for 1 hour of consumption for each droplet launched. To give you a guideline on typical tests sizes and cloud costs:
1 server with 10,000 users = $0.06 5 servers with 70,000 users = $0.60 10 servers with 150,000 users = $3.57