Here a link of DigiKey for the fibre optics prices. In my opinion, it not that bad… http://search.digikey.com/scripts/dksearch/dksus.dll. The type of cable is the 1000 um type micro-meter or 1 mm type. with plastic coating, the size is about 2 mm. As for the transmitter and receiver, check IF-D92 - transceiver and IF-E92A - transmitter. Here the site of Industrial fiberoptics → http://www.i-fiberoptics.com/ and here my tread about the use of fiber optics. http://arduino.cc/forum/index.php/topic,89595.0.html
The raison I prefer fiber optics is : - not affected by electrical noises
- simple to impliments
- simple interface circuit
One Arduino can do 7 sensors, but note thatevery sensor needs GND, 5V and a data line. As GND and 5V can be shared that means 9 lines in total.
As robtillaart mention, my question to you : How far the sensors will be ( the 7 sensors ). The farther the sensor and the Ardiuno, the worst the reading will be. It is recommend the sensor wire be as short as possible. You may have to build 7 sensor-Ardiuno, and 7 cable to the main Ardiuno for Display and data processing. Or one sensor connect to an amplifier circuit - booster ( 7 booster in total ) using a telephone wire ( 4 conductors ) to a main amplifier / adjuster circuit to compensate what in vs what out. The Ardiuno - sensor send data in a form of protocol ( ex: start1-data1-end1-start2-data2-end2-…the rest ) through the fiber optic - one long fiber optic, the receiving Ardiuno will process and display the data.
Anyway, just brainstorming here…
Here a picture of the connector and cable I am talking about.