They started pretty similar, and have traded in expensiveness depending on the year and which size phone you're comparing. These are all base storage size prices in USD, ordered from cheapest to most expensive.

2016: Pixel -> $649; iPhone 7 -> $649; iPhone 7 Plus -> $749; Pixel XL -> $769

2017: Pixel 2 -> $649; iPhone 8 -> $649; iPhone 8 Plus -> $749; Pixel 2 XL -> $849; iPhone X -> $999

2018: iPhone XR -> $749; Pixel 3 -> $799; Pixel 3 XL -> $899; iPhone X -> $999; iPhone X Max -> $1099