javascript price lookup calculation

I am creating a javascript function (using only pure javascript)

That will read in a dynamic quantity as a variable and calculate the price depending.

The pricing will be tiered. The tiers are between 2 - 20, 21 - 50 and >50.
There is always a base price of 1.99 added, then the additional costs are determined by the tiers.
2-20 = 0.99
21 - 50 = 0.50
>50 = 0.25
qty            price
2            1.99   (base price)
4            1.99 + (2 * 0.99)
            base price + additional qty charged at lower price
6            1.99 + (4 * 0.99)
...
20            1.99 + (18 * 0.99)
22            1.99 + (18 * 0.99) + (2 * 0.50)
...
50            1.99 + (18 * 0.99) + (30 * 0.50)
...
78            1.99 + (18 * 0.99) + (30 * 0.50) + (28 * 0.25)


This is how the price gets worked out and I just need to create a lookup function that reads in the qty and returns a price based on the above.