Convox has the same goal of a PaaS: to give you and your team an easy way to focus on your code and never worry about your infrastructure.
One big difference with Convox is that we accomplish this with single-tenant AWS things. You and your team's deployment target is an isolated VPC, ECS (EC2 container service), and ELB (load balancers).
If you're asking for a cost comparison, we're building Convox to be extremely cost competitive by unlocking AWS resource costs for everyone.
Its easiest to compare the cost of memory across platforms, though not always apples to apples...
The base Convox recommendation is 3 t2.smalls which is 6 GB of memory which costs about $100 / month. If your app can be sliced up into 512 MB processes, you can easily run 10 processes, which could be 2 to 5 medium traffic PHP apps on the cluster.
I'm finding PagodaBox pricing calculator a bit confusing but 6 512 MB processes, so 3 GB of memory, is $189.