I use a very affordable Manfrotto Monopod that was approx $100.00, solid aluminum.. no fuss.
If you want a head on it, no need for a big ball head, get a tilt only head.
The Manfrotto Bogen most certainly will hold the 500mm, I use it often. It's Approx $30.00 and it is even recommended for this purpose by the makers of the most substantial ball heads on Earth!;
EDIT: Since the time this was written many far superior tilt only heads have been introduced. Starting with the RRS offering, now Kirk, Sirui, and others have joined the fray. i now use the affordable SIRUI L10 and would agree with others that I was asking far too much of the Mnafroto.
If you plan to use a QR system (recommended) there is only Arca to look at, so the RRS clamp or one from Kirk, Wimberly etc, is the way to go.
Also consider replacing the foot of the 500mm with one that has the Arca QR machined right in,. rather than adding a lens plate.
What on earth did you plan on getting for $800.00?
Seriously, I use the Monpod lot of the time, but not as often as the tripod. Having Both available is best