Base and Unit Pricing Tiered Pricing

I have a product that has a mixed pricing structure. At 0-20,000 units we have a flat price. Then after 20k units we have a per unit pricing structure. So for example a 50,000 unit purchase

Base price = $100

unit pricing for +20k = $.1

So a 50k order is

100+(50,000*.1) = $5100

Currently we have to have two different products, a base price and a unit pricing product. I would like to have a tiered pricing structure where if the quantity is sub 20k then it uses the base price. If it is over 20k then it is the base price + the unit price. so it is kept under one product.