First off, you can get a replacement one for about $25 +shipping from electricscooterparts...
If your LBS buys one from Currie, I'm sure it will cost $50+ and don't know if it will be full twist or not..

To replace the throttle is quite simple...
On the throttle is a little "set screw" that holds it in place.. A couple of turns with the correct bit type will loosen it up to be able to be taken off..
(P.S. Make sure the BATTERY is DISCONNECTED (not just key "turned off") BEFORE you do any of this! - disconnect battery and turn throttle to make sure bike doesn't move!)**

Once off, somewhere along the throttle line, you can just cut the wire {forget about the connectors, they NEVER match** (also, remember, battery MUST be disconnected before you do this!!! - and preferably after a few minutes have passed). There will be FOUR wires inside..
1.) Red. Red is the main +5v power line to the throttle.
2.) Black. Black is the main ground wire.
3.) And 4.). There will be two other wires of any different color schemes..
One of those is a variable (~ 0-4.xV) line for throttle to controller control.
The other is a main 24/36/48v power line from controller to throttle to light the LEDs on the throttle.
(whatever voltage your bike is - oh and remember to buy the correct voltage led throttle)

Here is the part that is "tricky"..
First you will need a volt meter...
In order to figure out which wire is which... (at this point, it would be a good idea to connect red to red and black to black and have the other two wires taped up so they don't contact each other and especially the black wire)
With the red (taped up) and black connected (untaped, but attached - make sure it's not touching any other wire(s)), you re-connect the battery...
(P.S. If the black is not connected when you re-apply power the bike will take off at full throttle!)

With power supplied the two remaining wires will be variable (0-4xV) and main power.. Take your voltmeter and place black probe on black wire and red probe on one of the lines..
One line will read 24/36/48v and the other will read about .5v and at this point if you turn throttle, you will see the voltage vary between that at 4.xV. (P.S. Remember, Do NOT connect either of the remaining (2) wires between controller and throttle YET!)

Just make sure on the NEW throttle they tell you which is the variable line and which is the main power/led line.

IF you hook up the main power line (from controller) to the variable line (on throttle) you WILL blow the throttle and most likely destroy your controller, so make sure the correct ones are connected.

Once you have all wire connections figured out, DISCONECT the battery again, BEFORE you (re-connect) the respective variable and power lines.

If you have any other questions, feel free to ask.