http://arstechnica.com/articles/culture/wii-motion-sensor.ars/1

"When you think of gyroscopes today, though, especially with regard to heading information, they're around $300,000 and are capable of accuracy of ten-thousandths of a degree per hour so that a 747 after an eleven hour flight can land where it's supposed to land," explained Virginia. "Game controllers, such as what Nintendo has selected, don't need that kind of accuracy. What they were looking for something in the area of one-tenth of a degree per second. [The IDG-600 gryo in the MotionPlus] measure up to 1500 degrees per second; it offers accuracy and full-range motion. We worked towards [a cost of] $1 per axis."

This should mean each gyro will cost Nintendo about $6.  The rest of the WM+ components and assembly would probably only cost $4, so I would guess the costs for the WM+ would be about $10.  I think a MSRP of $19.99 is likely for the WM+ assuming Nintendo releases a standalone SKU.

Does this sound reasonable or did I read this incorrectly?

Sounds reasonable ... I was (personally) expecting the Wii MotionPlus to sell for between $15 and $20.

One thing I personally wonder is how much of a volume deal can Nintendo get? I would assume the $1 per gyroscope is based off of a volume of 10,000 to 100,000 units, when you're talking about buying 10,000,000 to 100,000,000 you should be able to get a better deal.

Sounds plausible I think 20 dollars would be fair, in fact I'm expecting that to be the case, but they could get more out there if they sold them for 15 like squrriel said and more Wii Motion + adapters that are out there the more games that will support it and hopefully it will become standard.

I'm not sure if the cost was InvenSense's cost, what they would charge Nintendo, or what they would charge a smaller customer.

Nintendo's volume deal will also be greatly effected by if they decide to put these in all future Wiimotes instead of keeping it an accessory.  As an accessory they will probably sell 20 million depending on the amount of support developers give the WM+.  As a built in feature in Wiimote ver. 2 they would probably buy 200 million (assuming 100 million more Wii's sold lifetime with an attach rate of 2 remotes per console).

If this is the case, why is Nintendo charging $30 for it? Is it bundled with something?

An axis is typically considered as having positive and negative, so there are really only 3 axis', not 6. Sony calls their controller a 6AXIS for marketing purposes, it really only detects motion in 3 axis'. So the motion + only needs $3 sensors. Everything else costs pennies so any major additional costs would be packaging, shipping and retail profit margin. Knowing all this, I would say $10 would be fairly optimistic, 15 realistic and the sweet spot, and 20 or more Nintendo giving us the shaft.

You're thinking of Wii Speak.   Wii Motion Plus doesn't have an announced price yet.

I won't pay more than $15 for a stand alone unit.

Nunchuck? $20

Classic Controller? $20

I bet that this thing will cost $20

Maybe, but it is an uneccesary attachment. You need a nunchuk to play the greater majority of Wii games. You need a classic controller to play the greater majority of VC games. If Nintendo wants this device to sell you will either A need it for the greater majority of future games OR B it needs to be cheap.