This is a convenience function, which returns just the world price
vector. Calls `gk`

and returns only the prices.

## Arguments

- P
An m x n matrix (or dataframe) of prices (row country, column goods).

- Q
An m x n matrix (or dataframe) of quantities (row country, column goods).

- pop
Optional m-vector of population numbers (if Q is per capita).

## Value

An n-vector of prices.

## Examples

```
P <- matrix(1:6, ncol = 2)
Q <- matrix(2:7, ncol = 2)
pgk(P, Q)
#> [1] 0.4051666 1.0000000
```