The only real cost is time for building a SaaS company. You can pretty much get by without paying for anything. Granted, buying the premium versions of certain products makes life a lot easier, but it's by no means required. Of course, as you launch, you'll likely have to pay for some server cost, but even then, you'll most likely be covered by credits for the first year.

For B2B SaaS, your first big costs are going to be advertising. Even just running ads against your own company name is a few hundred per month with Google. At that point though hopefully you're generating enough revenue from a few customers to cover the cost.