"Take a 50-100 meter diameter nickel iron asteroid in the asteroid belt (calculate its mass)- That would make a good WMD.... now calculate the energy to drop it on earth from the asteroid belt... "

Let's make it have density of 7.
50 meter sphere = 65,449.79 cu meters Mass: 458,148.5 tonnes
100 meter sphere = 523,598.33 cu meters Mass: 3,665,188.3 tonnes

Having it in asteriod belt is wrong location. If you gave this rock say 2 km/sec delta-v it would take months to get to a target. And rocket power required would be enormous. Having this rock as a earth crosser [NEO], one might have it require less delta-v to move to hit Earth- say less than 1 km/sec, but you have windows on a yearly basis. Could have 12 them so you have window on monthly basis, or hundreds I suppose.
Put it in an Earth L-point and with less than 1 km/sec and you have window every 3-4 days and can hit anywhere on Earth.
If you going to put in L-point, then why go with a natural rock. For instance make it out of depleted Uranium: [/kg m-3]: 19050. Compared to iron: [/kg m-3]: 7874. On Earth, Uranium is abundant: "Uranium is surprisingly common and is more plentiful than mercury, silver, or cadmium in the earth's crust, and is about as abundant as molybdenum or arsenic." http://www.webelements.com/webelements/elements/text/U/geol.html

Therefore Uranium in space also should be fairly abundant and if you processing Uranium in space for reactor purposes, you will have lots of waste in the form of depleted Uranium. If you aren't processing Uranium is space, but are mining, metals such as gold, platinum, etc, then you will also have waste in form of heavy element, such as lead and unprocessed uranium [this is obviously more radioactive than depleted uranium- but it's no big deal- lead would probably be bigger environment problem].
Ok, now, instead spheres, we want cylinders. 2 meter diameter by 10 meter long = 31.4159 cu meters. With density of 19 gives mass of 597 tonnes. Too heavy, thats 10 payloads of B52. Cut it in quarter, so there is 149 tons of this heavy stuff. Now you need a tail so this flys straight. This can simply be the rocket that powers this. Say it's somewhere around 2 meter diameter by 10 meter long, it's average density would be far less than 1 and weigh less than 31 tons, making the center of gravity in the section with heavy stuff. The gravity gradient will force this heavy end to point towards the center of Earth.
So this thing will be going around 10/sec.
With lots of rocket power, say add another 10 meter section, you could shorten the window. If you trim your 149 ton payload to say 100 tonnes, you could say have around 10 km/sec added, giving a velocity of around 20 km/sec at impact.

More details. Now, you could put this in GEO in three locations and be able to hit anywhere on Earth in less than a day. From point view of President picking up the phone and saying hit this target, GEO gets the target hit in shortness period of time, but comparing to an intercontinential or ship launched missile gives about a hour. On Earth the further a missile could travel to any point in the world is around 20,000 km and with a number launch sites one shortens this to a few thousand miles. Whereas nearest point on Earth to GEO is 36,000 km.
Now nearest L-point is Earth/Moon L-1, and which is about 75,000 km from the Moon. And since the Moon is 384,400 km from Earth, that means it's about 300,000 km from Earth. But of course L-1 isn't a point but it's an area, so one could be say, 250,000 km from Earth and still be in L-1.
Which leads to other aspect and that is not only are L-point not points but gravitationally they nodes in vast highway. All L-points in this solar system are "connected". The "paths" that take you to various Ls may be slow and can be time dependent, but for instance the path between Sun/Earth L-1 and L-2 is open most of the time and one route goes close to Earth. Anyways, the Ls might not be a good location if you suddenly out the blue want to strike a target. But with planning one could arrange to have the weapons at a place and loiter to some degree and be able to hit anywhere on earth in time scales of less then an hour. So depend on number assets, one could have numerous windows, for instance during few hour period or most of day, one could have the go-no go decision which could hit it's target in less 10 minutes at shortest or up an hour or so. With windows expiring the weapon returns to L-point to top off it's fuel used for "station keeping".