First of all you need to decide scalability type you want to use for your module. I think that you should take a look to Basic Scaling (https://developers.google.com/appengine/docs/java/modules/)
Next, to process tasks from pull queue you can use Cron to check queues every several minutes. It will be important to request not basic scaling module, but frontend module, cause cron will start instances. The problem is that you will still need to pay for at least one instance of frontend cause your cron job will not allow it to shutdown.
So the solution could be the following:
- Start cron every 1 or 5 minutes and call frontend
- Check queue in frontend and issue URLFetch request to basic scaling module if there are tasks in pull queue
- Process tasks in queue using basic scaling module
If you use F1 instances for frontend and b2 or greate for other modules it could save you some money.