Ask HN: How do I bill my client?

The prospective client is creating a product and have decided to outsource their product development.

As a company, we decided to give it a try so that we can fund our own product which is under active development. Having said that, we are unaware of the economics of project development as we all have background in SaaS product companies.

I have come to the conclusion that we should charge them on manpower basis so that we don't get slowed down i.e $x per developer per month + $y where x is subjective to skills and experience and y = cloud infrastructure cost.

This is in contrary to the hourly task based approach.

Please advise.