You can go to any number of online retailers and buy a wireless keyboard with trackpad for <$20. Just without a brand name written on it.
You people act like you don't know thickness and trackpadness.
Yes, and it must be noted that you get what you pay for in many cases.
I have had those <$20 keyboards, and they tend to stop working in fairly short order, and don't stand up well to abuse. They also are not backlit, and do not provide protection for the surface screen when the surface is not in use.
This isn't just a keyboard/trackpad, it's also a cover for the surface that magnetically attaches, so it's fast and easy to hook up. You pay for convenience as well.
I've got a backlit keyboard on my laptop. Had to pay extra to get it, but it was well worth it. If I picked up a Surface 3 or Pro 3, I'd definitely get the type cover.
Picking up a <$20 keyboard will also force you to pick up a different cover for travel, plus a travel bag that can now carry the bulkier keyboard and the surface. You'll lose time and productivity having to pull out a separate, non-attached keyboard when you need it. In the long run, for a business user or for a student, the type cover is the best way to go.
If you can't really afford the type cover and want a bulkier, cheaper keyboard, then you need to reconsider your need for a tablet, and a small 2 in 1 that is less expensive would probably be a better choice for you.
Find the tool that works best for what you want to use it for. While the Surface is nice, it's not necessarily for everyone. Budget minded people are generally better served finding a nice 2 in 1 or less expensive tablet combo setup. While I had a budget to pick up a Surface Pro 3 a while back, it's not what I went for. I wanted a larger display, so I got a 15.6" 2 in 1 laptop. I get the benefit of a larger touch screen tablet and a powerful laptop with plenty of onboard storage, and a full size, backlit keyboard with 10 key pad. Definitely better for games and such, as well as better for working with larger spreadsheets.