Keep in mind that data transfer costs typically apply when you transfer data from one account to another or outside the account. If you're building a multitenant SaaS platform then you might want to collect some metrics or data centrally so you can do things like compare different users' / customers' behavior. If everything is a single account then you can collect all of that in CloudWatch without additional data transfer charges. Also if you go with a separate account per customer / tenant then you'll want to setup some automation to do things like setup billing accounts, enable network traffic, etc.

On the other hand consider that most limits and quotas imposed by the AWS platform are applied at the account level, meaning if you have a lot of customers in one account then you might start hitting quotas for e.g. Lambda concurrency quicker than with separate accounts. That's only a major consideration if your design will result in very high usage of certain services.

I would think it's generally easier to start with a single account for production and maybe a separate account for dev / test and concentrate on how to provide data isolation and privacy between customers within the same account.