Instruction: Given a question and the related document, and generate the answer to the question based on the information provided in the document.

Input:
Question: What would a second personal device cost in addition to the orig. device and charging unit?
Document: This device works well and enables me to use my own headphones to hear. Using FM transmission rather than infrared is a big advantage, because line of sight is not required, and there is no signal interruption. Not a cheap device, but I think we'll worth the cost.
Ordered as a gift. Item wasn't compatible with hearing device and needed to return. Would have been helpful to have more information on compatibility before ordering. Shipping charges were costly.
And for the substantially hearing impaired such as myself, the SET840-S transmitting unit has multiple adustable settings for treble-boost and sound-compression to enhance speech intelligibility, and the Body Pac receiving unit also has a convenient volume control and a left/right audio balance control. I would note, however, that this fine instrument does have a limit in the volume of the audio it can reproduce.
A real boon for the hearing impaired, although it would have been nice if it included a set of earbuds.The charger that was initially sent to me was not able to charge the device due to a poorly designed connection that was designed to accommodate multiple international plug styles. I called Sennheiser directly, and they where very helpful, sending out a replacement immediately without even asking me for any proof of purchase. The replacement had a much better design with firmer contacts and works fine.
I did some research and am so glad I did. This unit is awsome and has saved me from not watching tv or playing video games. The first time I plugged it up I could not believe what I was hearing....EVERYTHING. I now can play games,watch movies and hear people wispering, footsteps,birds, you name it. I could never hear this before. I would highly recommend this unit to anyone who has a hard time hearing.
I wish I had known about this product years ago. I did not know what I was missing.The Sennheiser SET840-S cost a little more than the top of the line TV Ears, but it is well worth the difference in price!
This device  has improved my ability to hear normally by almost 100% ..Nothing will bring back normal hearing 100% if you have lost it..Very happy with the results... Easy to set up..
My husband has an accoustic neuroma. Hearing aids don't help him so we got him this device to help him hear TV more clearly and so that I was not being blasted out of the room by the volume. It works very well.
My husband is very please with this purchase for listening to the TV. The quality of sound is so much better than his old unit.
Although this unit does what it advertises quite well, the sound quality is quite tinny. We tried all possible adjustments on both the unit and the television but it remained too tinny.
Answer: I talked with customer service from Sennheiser and was told about $179 for RR 840S which is the lanyard and $169 for RR 840 lanyard which is about $10 cheaper.  This was directly from the company but prices are similar from Amazon.  You will also need a second battery.  I have not looked into the price on that because we have 2 batteries.  My husband was given his by the VA and I love it so much we are fighting (just kidding) over who gets to use it so I am going to purchase another lanyard.