Taxes are generally not called out separately in a contract. Pricing specified in the contracts are almost always all-inclusive for all sorts of reasons. The most important of those reasons is simplicity. Because simplicity is very important to get the contract through customer’s legal department.
This isn't a legal advice, but some contracts address tax burdens in the reps and warranties section or at least in a specifically called out section.
For example, X assumes all tax burden related to this transaction, or X and Y agree to carve it up in this way.
It is best to just talk to your lawyers regarding about this.