It's often useful for sellers and buyers of used boats to know the approximate value of the installed electronics. A well regarded used marine electronics dealer is Max Marine in Pompano Beach, FL. The web site is http://www.maxmarineelectronics.com.
A comparison of the prices of their for sale items with the electronics of a used boat may provide some useful information. They seldom have the resources to answer emails asking for values.
They deal in fairly high end items so looking for a 7-inch MFD price may be disappointing.