Any taxes would already be included in the price.
We stayed at DVC on an exchange in June and paid tax on our 7-night QSDP, although the CC receipt is not broken down. (If the extra cost wasn't taxes, it was sure a hefty fee).
What you perceived as taxes may very well have been the $95 incoming exchange resort service fee that Disney/DVC charges to all incoming RCI/II exchanges, whether or not you have the Dining Plans.