The prices for Gold you find while playing are in part set by the region you live in. Certain regions require the charge of VAT/GST for purchases, and can also be affected by the current strength of your currency against the US Dollar. So you may notice the price for Gold will fluctuate on occasion depending on the region you live in. Both of those factors appear in response to changes in the global economy and are not something we can control.
Additionally, the point at which any taxes or additional fees are listed can vary depending on what platform you're playing on. Some platforms show the final price upfront, some do not add up all costs until the final payment screen. If you are noticing varying prices between platforms, it is likely that fees and/or taxes are being added at different times in the purchase flow.