In 2007, a science working team published a report on possible science goals for a twenty-teens Mars orbiter and possible small lander. In that report, they discussed a number instruments and their estimated costs. This is the only place where I have seen costs listed for such a wide range of instrument types. The goal of the study was to narrow down goals and instruments to a manageable cost. The result was a proposal for the Mars Trace Gas orbiter, which is now scheduled as an ESA/NASA mission for launch in 2016. At a minimum, that mission would fly the first three instruments on the list for a total instrument cost of ~$71M based on the numbers in the report. There is a strong desire to add a high resolution imager with an instrument cost of $25-45M plus additional costs in the spacecraft.
A couple of caveats. These costs are rough estimates; costs can go up or down by adding or removing capabilities. Also, these costs are likely in 2006 dollars; they likely will rise substantially with a decade's inflation.
Still, I found this list useful in understanding some of the trade offs for mission planning. The saddest trade off I know of was the deletion of the magnetometer from the DAWN asteroid mission when that mission had cost overruns. Theories suggest that the Ceres asteroid may have a subterranean ocean. The magnetometer would have told us by measuring changes in the solar magnetosphere that would be caused by such an ocean. (This was the method used by the Galileo spacecraft to detect oceans in the Galilean moons.) Now, it is probably at least decades before we will know for sure.
$35M/42kg: solar occultation Fourier-transform infrared spectrometer - Measures trace gases in atmosphere
$35M/35kg: Sub-millimeter spectrometer - Measures trace gases and temperatures
$1M/1kg: “MARCI-like” Camera - Wide angle camera for observing weather patterns; similar designs have flown on previous orbiters
$12M/10kg: Thermal IR or “Lite” spectrometer system - Measure atmospheric dust and ice; could also do spectral mapping of surface composition depending on design (and cost)
$45M/85kg: “HiRISE” Class Imager (HCI) - High resolution imaging of surface at around 30 cm/pixel; would largely be a relight of the highly successful Mars Reconnaissance Orbiter HiRISE camera (which keeps the costs down). Any high resolution camera adds other costs to the spacecraft to provide accurate pointing, an ultrastable platform, data storage, and communications.
$25M/20kg: 1-m resolution camera - lower resolution than HiRISE, but would cover larger areas
$5M/1kg: Ultrastable radio oscillator - Gravity measurements
$30M/32kg: Multibeam Lidar Altimeter + Ultra Stable Oscillator - Measure growth and recession of polar ice caps
$30M/30kg: Multi-spectral SWIR/TIR [short wave infrared/thermal infrared] or “Capable” Spectrometer - measure ice and mineral compositions at ~100m resolution
$40M/45kg: Synthetic Aperture Radar - measure terrain up to 3 m below covering sand and soil. Costs and weights assume radar shares the craft's 3m communication antenna; otherwise, an additional 30-40kg and unspecified cost required for a dedicated antenna.
$5M/3.2kg: Landed Meteorology - local pressure, temperature, winds plus profile of atmosphere above lander via upward looking spectrometers and LiDAR
$10M/1.5kg: Precision radio tracking - geodesy measurements
$5M/2.4kg: Heat flow instrument
Resources: Mars Science Orbiter (MSO) SAG Report http://mepag.jpl.nasa.gov/reports/MSO_SAG_report_071006.pdf