NClabzDo that, and you're likely spending more money, at least compared to the lowest price. Anything smaller than a Model M, now you're going to have issues fitting it all inside the stock case. Don't forget the battery, the charging circuitry, etc, etc, etc.
At retail, this thing is overpriced. At lowest, it is a bit overpriced. But, someone else did the design work, it is PS/2 or USB compatible, it has a battery, it has a charging circuit, it has all the parts. And it is in a relatively clean package (albeit a large one.)
See here for an example of the bluefruit being used for just this:
https://learn.adafruit.com/convert-your-model-m-keyboard-to-bluetooth-with-bluefruit-ez-key-hid/