Even the cheapest UAV-explosive is more expensive than the explosive without the UAV. A UAV that's "cheaper than a handgun" [0] (as in the grandparent post) is, at present, going to have a payload of perhaps a couple of ounces. How many feet of lethal radius can you get from a couple ounces of payload? How "very very close" can you actually manually fly a UAV with a small bomb attached?
Being able to kill someone with a disposable weapon, and not be spotted by onlookers or security tapes, is still potentially a security nightmare. I'm just not convinced that "cheaper than a handgun" is the present or near-future reality for such a weapon.
[0] cheap handguns are under $250, and rifles can be under $200. Prices taken from http://www.impactguns.com/
The simplest explosive would probably be a single .380 or 9mm pistol round with the primer replaced by an electric igniter. In the field when a gun back fires it often seriously injures the person holding it and people near the weapon. The NATO 9mm pistol round [1] weighs less than 20g and has 600j of energy. Probably wouldn't use the lead bullet. I can imagine (as I am sure others can) an enclosure for this which would not increase the weight significantly but would convert it into an omnidirectional device.
I don't doubt for a moment that you could seriously injure an unarmored individual with something like that within 2 or 3 feet.
The big issue is that 'terrorism' is not about inflicting harm, but fear of harm.
A few quad rotor UAVs outfitted as you describe, possibly even with frangible rounds or just dropping tiny flechettes such as those used from the air in WWI, would easily cause panic.
For that matter, "suicide" UAVs that are either capable of delivering a lightweight shrapnel load without concern for the vehicle's survivability, or just using the kinetic energy of the moving vehicle and the rotating blades to crash into crowds, especially in some sort of flock, would discourage a lot of people from gathering in public places, going to clubs, sporting events, malls, tourist attractions, etc.
Sorry, I'm not from the US and with a cursory google for gun shops found that a Glock pistol of some description (a brand I recognise) is ~$500.
If you have access to basic tools (as in, hacksaw and glue), you can build your own fixed-wing airframe using $100 of foam, carbon rods, and off-the-shelf motors, ESCs, batteries & props & servos[2]. See RCGroups for hundreds of downloadable designs. Your radio control gear will cost you about $40 for the cheap stuff[3]. An ArduPilot 2.5 autopilot is $180 off the shelf [4].
The thing is, if you are building your own scaling up (within reasonable bounds) doesn't really change the price. Something Skywalker sized (~2m wingspan) is easily build-able by almost anyone and will easily take 1kg of payload. $120 dollars[1] gets you a video link & camera with a 20-30km range you can use to fly the thing straight into whatever you like.
So the dumb GPS targeted version is 100 + 40 + 180 = $320
The video targetted version is 320 + 120 = $440
Obviously, that doesn't include the 1kg of 'payload'.
Being able to kill someone with a disposable weapon, and not be spotted by onlookers or security tapes, is still potentially a security nightmare. I'm just not convinced that "cheaper than a handgun" is the present or near-future reality for such a weapon.
[0] cheap handguns are under $250, and rifles can be under $200. Prices taken from http://www.impactguns.com/