Ideas for a price tag on parts from old city set?
Bricklink price-guides:

The trapezoid:
https://www.bricklink.com/catalogPG.asp?P=55767&amp;ColorID=86

The smooth plates:
https://www.bricklink.com/catalogPG.asp?P=48288&amp;ColorID=85

Don't know what you mean with modified tube, but this part might be considered a tube and unique:
https://www.bricklink.com/catalogPG.asp?P=40244&amp;ColorID=85

Not exactly the most expensive pieces, so I think you'll make more if you sell the set as a whole rather than parting it out...

The Trapezoid price surprises me a bit as I think it is indeed somewhat unique, it appears in 3 sets but the other two sets it appears in, it appears in orange. 
I would have thought it's a wanted piece as it can be used to make bridges in train layouts...

The large tiles, I wouldn't pay even the rather low bricklink price for them as I have gotten hold of quite a quantity of them when last visiting Legoland Germany, where they were available in bulk at the "Fabrik" shop.