You can use usage with Pay-as-you-go ovDCs.
Make sure that the billing policy that your cost model is using is specified as "usage" for CPU. Also make sure that you have a rate specified for the CPU resource for the reporting period.
Finally, for testing, you have to make the vApp use some CPU cycles (use cpuhog or something like that) and make the reporting period short, and/or the price artificially high so that the cost and usage do not round to 0.
If you do all this you should see non-zero usage and cost for the vApps that used enough CPU cycles.
Cheers,
Milos