I tried to connect a Logitech K400 wirelesse keyboard with no success. I bought a female-female usb adapter and plugged it at my ASUS usb docking cable. The RF receiver isn't recognized and the keyboard can't be used....
I'm not that surprise this isn't working. One, the keyboard is not blue tooth, which you know. Two, you need drivers to work the unifying usb receiver. I would be surprised if Android had the drivers. Didn't you notice when you plugged into your PC, WIndows or OSX downloaded drivers to install?
To your point, no you don't have buy the ASUS keyboard dock for $150EU/US. If don't mind it not matching, from what has been explored in other threads, the Transformer Prime (TF200) dock is the same, and can be found for around $100EU/US. But since you're you were trying to use something else, there are a plenty of other blue tooth keyboards that can be used. You're looking for one with the HID protocol. Although, you can probably use the SPP, but you'll probably need to make sure you get the drivers somewhere.
I suspect you're looking for that wireless to be able to use with it. I only briefly explored it for use with my phone. You'll have to search and research. There may be something. Personally, it wasn't a must have. Since I'm right next to the tablet, I can just touch the screen just as easily. Other than a few games on OnLive, I can think of what it can do better than my finger on the tablet. Plus, I think the cost will be the same after both the keyboard and mouse/touch pad.
I guess the question for you, not so much is ASUS keyboard dock is pricey, but can you do what you plan with something else. And is the cost of whatever you get, worth the benefit derived.