Dave and Illusionist thanks for your great posts, I have visited the
https://pcbshopper.com/ site where you can get prices from several places at once and it looks to me like the difference from 2 to 4 layer is about 3-4 times the price, while the thickness does not really matter.
The solution with using 0.8 instead of 1.6 sounds great but do make the board thinner, is that why some recommends a 4 x 0.4 layer board instead of a 2 x 0.4 layer, to make them less flimsy?
My 4 boards has to be mounted on the side (4.7" x 1.5" - / - 120mm x 39mm) with the same spacing as standard BNC's on a scope is mounted (is there a standard spacing, what is it?) I do not think that 0.8 thickness is a problem here?
The trace design uses Coplanar Waveguide, track width 1.27 mm, gap width 0.2 mm. I do not know the Dielectric Thickness or constant but is that posible just to add in the notes as something like "Please select a FR4 board where a coplanar waveguide, track width 1.27 mm, gap width 0.2 mm gives a 50 Ohm impedance +/- 1 Ohm"?
By the way, I have found a graph, showing losses when comparing a FR4 and a Rogers PCB. Am I right in that thay are nearly the same for an <1GHz 2 1/4 inch (57 mm) trace on a probe board?