Using VPC Peering in addition to Transit Gateway for cost optimization

It's (always) a trade off between cost and complexity, so it depends.

In this case the additional cost arises just from the TGW attachment cost, which (depending on the use-case) can be marginal. Customer should do a calculation to understand the financial advantage and factor in the added complexity as well.

Then, it will be up to them to decide whether or not the financial advantage justifies the added complexity or not.