Hello,
Quotation comparison in standard SAP is on base price only. The logic is that the taxes should remain the same for all vendors and can be added on later. This is a drawback in scenarios where the tax rate changes with where the vendor is located.
Generally, custom reports/ forms are developed based on the data in ME47 and the comparison is done out of the system in such a scenario.
Please have a look at this thread, this has been discussed:
However, if you are using BR localization, this feature is available.