THE PRICE OF GREATNESS
The Surface Pro 4 has a US$899 starting price, whereas the most affordable Surface Book costs US$1,499. That may look like a big difference, but it's not a fair comparison because the entry-level Surface Pro 4 only has a Core m3 processor and 4GB RAM.
The Core i5 Surface Pro 4 with 256GB of storage and 8GB RAM costs US$1,299, whereas a similarly-spec'd Surface Book costs US$1,699. However, note that this Surface Book doesn't have discrete graphics; if you opt for it, the price shoots to US$1,899. Prices go up pretty quickly after that. A Surface Book with a Core i7 processor, 8GB RAM, 256GB SSD and discrete graphics costs US$2,099; and this is before maxing RAM and storage.
Finally, note that the Surface Pro 4's Type Cover is an optional accessory and will set you back US$130. This closes the price gap further, especially if you were to pick up a Surface Book without the discrete GPU.