Two of them being at 0.5 brings the total to 1.5 vCPU, which over an entire month adds about another 375 compute hours for an extra $60, which is still much low

Two of them being at 0.5 brings the total to 1.5 vCPU, which over an entire month adds about another 375 compute hours for an extra $60, which is still much lower. Indeed autoscaling seems like it could be the cause. According to the documentation that's a setting you can configure per "compute", but I don't know if it's the default.