Tiered Pricing Model

I have a product for On-Location Headshot Sessions.  I want to implement a per-person billing with price going down in tiers.

An initial $xxx covers travel, set up, and the first 4 individual headshot sessions.

The 4-9 people are billed at $100/person

10-19 people are billed at $90/person

20-29 people are billed at $80/person

This is not my actually pricing, but a value was used to better explain what Im trying to do. 

The first 4-9 people at $100, employees 10-19 billed at $90 each and so on. 

Any advice as to how I can create a booking product with this type of pricing structure?

Thank you.

I do a base rate and includes one portrait session for set up then use a drop down additional menu. And slowly discount per person. Problem is in company's you often have someone on vacation or sick or a person from another department wants to join in. so all my final billing is done after the session. At least they have a rough idea on cost and only pay the deposit and do not get gallery access until invoice is paid.