I actually came up with a few different brooches. Every single one is uber easy & cheap, totally in keeping with the ideal behind the challenge. The first type is totally homemade -- I call it the "button brooch".
The basic materials for creating a simple button brooch are:

buttons, 4 or 5 sew-threw and 1 shaft type
a thin, flat wooden disc (available at craft stores)
a pin back (also available from craft stores)
also needed but not shown:
paint or permanent marker to color the disc background
regular glue to fasten the buttons to the disc
optional stuff: rhinestones, beads or other baubles
Once you see the finished product it becomes pretty obvious how simply this was created. I think the fun in this is how imaginative you could get for relatively little money -- there are soooo many really cool buttons & beads out there that the possibilities are endless.

So, here is a picture of all the button brooches I made, each one is very different (you can tell the one on the left is made of the materials shown)

The next brooch I created by taking a piece of broken jewelry and simply gluing a pin back to it. Some of my readers may remember my Etsy destash loot -- well, I used a piece of a broken bracelet to create this next brooch in less time then it takes to pin it to a sweater, lol:
Here's the broken bracelet piece:

The pin back glued on...

...and the final brooch!

Finally I have 2 pins I made from pendants. This was just as simple as could be -- glue a pin back onto a pendant and you have the option of wearing it as a necklace or a brooch.




Of course there are endless ideas one could come up with ranging from delicate artistry to basic arts & crafts 101 and everywhere in between. The ones I've made here certainly fall in the latter category, lol, but that's what I like about them -- they were quick, easy & inexpensive yet creative and fun, too. Plus I really do like how they look!
Thanks for the VERY inspiring challenge Kasmira!