The Surface Pro 4 does not have a dedicated GPU at all.
On the other hand the more expensive Surface Book (detachable keyboard) does have an optional nVidia 940m (if I am not mistaken). The base model sells for $1,500. But the least expensive version with the dedicated GPU is $1,900.
http://www.microsoftstore.com/store/msusa/en_US/pdp/productID.325716000
All things considered, the price difference between the Surface Pro 4 and the Surface Book is not too bad. The Surface Pro 4 with the 6th gen Core i5, 256GB SSD and 8GB of RAM is $1,300. Plus another $150 for the keyboard which brings the total to $1,450. As stated the least expensive Surface Book with the 940m is $1,900, but that price includes a higher quality keyboard which has a battery.
The problem with the Surface Book is that most of the battery is in the keyboard not in the tablet. While the Surface Book is claimed to last up to 12 hours... when separated from the keyboard the tablet part only has up to 3 hours of battery life.