CRM-15636: I think this patch fixes the issue.

I basically did a CRM_Utils_Array::implodePadded on 'extends'. But I am
not sure whether I did this at the right place (in the PriceSet BAO).

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* CRM-15636: Using the API to create a price set for both events and contributions.
  https://issues.civicrm.org/jira/browse/CRM-15636