You can charge for the software (your app) but you cannot charge for access to the service (QuickBase User Account)
http://www.quickbase.com/terms-of-servicehttp://www.quickbase.com/acceptable-use-policy"""
5.3 Restrictions.
You agree not to copy, sell, rent or sublicense (including offering the Service to third parties on an applications service provider or time-sharing basis), lease, loan, redistribute, or create a derivative work of any portion of the Service (except solely to the extent necessary for you to create applications permitted under Section 7 (�Application Development and Distribution via QuickBase Exchange�) of these Terms of Service), use of the Service, or access to the Service. Such restrictions do not apply to the Content you place on the Service. You agree not to access the Service by any means other than through the interface that is provided by QuickBase for use in accessing the Service.
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Trust me, I wish with all my heart I could do this. But they have to protect themselves, and their income stream somehow.