I wear two hearing aids with DSP processors built in. Let me tell you a little bit about why they are so expensive. The largest supplier of hearing aids in the USA is Starkey in Minneapolis. I've been to the factory, and have experienced the process from start to finish courtesy of the president of the company.

1. Because hearing aids, especially BTE and ITC types use a single cell 1.5 volt battery, which can drop as low as 1.3 volts through its useful operational life, the circuits must be of extremely low power consumption and low voltage. The only chip material that works for this is germanium, which has a diode junction breakdown voltage of ~ 0.3V as opposed to the ubiquitous silicon used in consumer electronics. While germanium was once very common for transistors and some early integrated circuits, it has fallen out of favor in the microelectronics world. There are only a handful of sources and companies now that work with germanium, thus the base price is higher due to this scarcity. You can't just take an off the shelf silicon chip and put it in these aids. Each one is custom designed in germanium.

2. The process of properly fitting a hearing aid is labor intensive. Custom ear molds must be created from latex impressions, and these need to be fitted for comfort. A small variance or burr can mean the difference between a good fitting mold and one that is painful to wear. Additionally, if the mold doesn't maintain a seal to the inner ear properly the hearing aid will go into oscillatory feedback. Sometimes it takes 2 or 3 attempts to get the fitting right.

3. On the more expensive aids, labor is involved in doing a spectral hearing loss analysis of the user's hearing problem, so that the aid doesn't over-amplify in the wrong frequencies. Just throwing in a simple linear amplifier is destructive to the remaining hearing due to the sound pressure levels involved.

4. Construction of aids is done by hand by technicians, especially with the popular ITC (in the canal) aids. At the Starkey company, a technician is assigned to create the aid from the ear mold, fit the chips and microphone/receiver and battery compartment, and connect it all with 32 gauge wire and make sure it all fits in the ear mold. This can be a real challenge, because human ear canals aren't often straight, but bend and change diameter. Imaging a room with a hundred technicians sitting at microscopes assembling these. Each is a custom job. There's no mass production possible and thus none of the savings from it.

5. After the aid is created, then there's the fitting. This process is also hands on. Getting the volume and the audio spectrum match right is a challenge, and audiologists have to have chip programming systems onsite to make such adjustments withing the limits of the aid. Sometimes aids are rejected because the user isn't comfortable with the fitting, and then the aids go back to the factory for either a new ear mold, new electronics, or both.

6. There's a lot of loss in the hearing aid business. Patients don't often adapt well, especially older people. There may be two or three attempts at fitting before a success or rejection. Patients only pay when the fitting is successful. If it is not, the company eats the effort and the cost of labor and materials. Imagine making PC's by hand, sending them out to users, and then having them come back to have different cases or motherboards or drives fitted two or three times, and software adjusted until the customer is happy with it. Imagine 4 out of 10 PC's coming back permanently after trial and error with a customer.

7. Early hearing aids weren't anything but simple amplifiers. Even until the mid 90's there was very little spectral customization. Now many aids are getting features like frequency equalizers and DSP noise reductions that we take for granted in even the cheapest silicon based consumer electronics. Hence, prics has increased with complexity, but there's still the high cost of custom special chips, and lots of labor.

So for those who think mass production techniques used on iPods would work just fine for making a delicately balanced instrument that must fit in your ear, please think again. As a hearing aid user since 1969, do I think the price tag of the special hearing aids today are worth the price compared to the simple linear amplifiers I used to have to deal with? Absolutely.

The NASA GISTEMP data you cite is polluted with questionable surface station data. Even the press is beginning to questions James Hansens methods in arriving at the data and graphs he distributes. Loys of "adjustment" going on. See this article from the UK Register

A much better metric is the Lower Troposphere temperature as measured by satellite. It in fact shows no trend from 1998, and also shows a big drop globally in temperature since January 2007. See this analysis of the satellite data.