Don't forget we live inside of a bubble in the experimental world that is somewhat skewed when compared to the certificated aircraft world. Remember that many of the members in this forum also own certified airplanes, and as such can be used to prices of items that reflect the additional cost of install into certified airplanes. $5K for a SL30 into a certified ship is very reasonable, not so much for the homebuilts. The simple reasons the values stay high is because the owners of certified airplanes can still pay a premium over what we pay and still end up getting a deal compared to new from a shop. Believe me, many times when working on certified airplanes the paperwork alone can sometimes take as much time or more than the physical work. Same with some of the services...a VFR transponder check for example takes much longer to do the paperwork then it does to do the actual test.
Anyway, using prices from certain online or printed catalogues/advertisements is also not always a good measureing stick of value on the finished or overall purchase/install. Many times there is more to consider.
That's just my 2 cents as usual!
Cheers,
Stein