Get a ballhead with an Arca-Swiss style camera plate. Some makers use proprietary plates. Manfrotto has three or four different plates that are not compatible with each other. Arca-Swiss style is the industry standard.
Here's an Induro ballhead at your price point:
http://www.indurogear.com/products/induro-bhd1.aspx
If you can, consider moving up at least one step in price/performance. For example, a 5D with a 24-105mm puts a lot of torque on the ballhead. That's not really a lot of lens. All other heads are slow to adjust and hard to use, so you really should get a ballhead.
Oh, if you're going to do video, even casually, you need a video head, but your budget is too low for that.
Dave