There really isn't a simple answer to this question... If all you're looking at is performance, you might actually be in the wrong place, but I'll try and answer as best I can in my opinion since I've had the same questions about each version of the Surface since before the launch of the first Surface Pro.
From my research, almost regardless of which tier of Surface you are getting, a desktop of very roughly equivalent performance will cost you on average about a third of that model of Surface Book, based on variables such as self built or prebuilt and what you want to call equivalent performance. Keep in mind here that we don't have any benchmarks on the Surface Book yet, although from my experience the Surface line tends to out-perform it's hardware by the theoretical numbers. If we look at a performance laptop, one of again roughly equivalent performance tends to run from half to two-thirds the price, depending on how picky you are about getting a brand that will last more than two years. That being said, the Surface line is about being far more versatile than any of those devices, so start adding in high-resolution monitors, stand-alone stylus pads/tablets and other accessories needed to give you an equivalent experience (ignoring weight and size somehow) and the price might get a whole lot closer to what you're paying anyway.
As for my own thoughts on which one to get after having been down this path already three separate times (and saying it that way doesn't even describe the wrangling I've done in my own head trying to decide what I really want, why does Microsoft keep doing this to me?!?), I have after much deliberation actually decided to go with the i5-8GB-256GB-w/dGPU model rather than a higher model for every day use and casual gaming, since I see it as the best values for the price out of the lot. After buying the biggest baddest Surface Pro I could afford for three generations in a row, I've finally reconciled to the fact that I buy Surface brand devices because I love the versatility, not because I'm trying to get 80 fps out of the latest game on decent settings. I've sold computers to consumers then to large businesses for the better part of ten years, and a balanced machine is more important to me than paying out the nose to hit the bleeding edge of tech. I guess what I'm getting at is that any of us that are still shelling out top dollar for a Surface for gaming might still be delusional. That being said, I feel like the Surface Book will certainly hold it own for casual gaming, and I am excited to be replacing my main PC with this very neat device.
P.S. Apologies for going longer than intended, hopefully the second paragraph at least was helpful in answering your question.
