Alexander,
G'day
As Brian has outlined through his summery of avionics replacement above, (as did I a number of posts ago - albeit without actual pricing

) avionics is
not the major concern when buying an aircraft...
don't, please don't... focus on the radios,
there are much more important issues to get your attention when buying an aircraft.
I'll say it again (as I can't stress this enough), the key to finding a good aircraft is in the Airframe and Engine, its not the Radios - especially if you're talking VFR. Good radio's are nice, and in a perfect world, all aircraft would be problem free and jam packed full of goodies - but the reality is, there not
Its not much good having the fanciest radios on board, yet the Airframe is rotting around you, or the engine won't get you up there to enjoy the experience. Both these issues will keep you gounded, and cost you a absolute fortune for a remedy....
not a radio as such, to go to an extreme for example, (depending on where you're flying) you don't even need to have a radio.....or could operate on a cheap hand held for example.....are you getting my point here?.
Lastly, as far as the value of an aircraft goes......what is it
really worth to you???? you're the one forking out his hard earned $$ here, and its you who stands to gain or more importantly, and possibly, loose the most as a result of your choices. A sightly over priced
but good plane kept for years and years is not really an issue as such, where as by comparison, one bought to keep for only a year or so, is likely to be.
Good luck...
Cheers