You don't need a ballhead on your monopod. If you do, use a two-way swivel head; they're a lot easier to work with than a ballhead when on a monopod.
I used to have my Gitzo 1588 monopod set up like
this (I didn't add the Arca-Swiss type clamp until a year later) but I ended up chucking the head. It just didn't offer enough for the drawbacks, as it was hard to keep tight with a 300mm f/2.8 on top and increased the height of the monopod, making it hard for me to sit and shoot without messing with the head to get the monopod at an odd angle.
The swivel head's real cheap, so you are correct in thinking Manfrotto or Gitzo for your monopod, and a cheap head. The Manfrotto 681 series is already more than sufficient for most gear, but I really like my carbon fiber Gitzo 1588 (near the top of your budget). The new 6x series is cheaper and better, so you can't go wrong.