Hi Mikael,

> is there a quick fix/hack that somehow allows me to get it look "almost like
> it should" in a small document today with bonum?

This might get you somewhere. I hope you don't have too many double
primes in your document...

NB: `%macros=mkvi` has to be the first line in the file.

Cheers,
Sietse

% macros=mkvi

\setupbodyfont[bonum]

% This macro takes 1 arguments, #upper. It postfixes a negative kern just a
% bit larger than the width of #upper, so that #upper does not push
% the next content away to the right.
\def\myprime#upper%
   {% 1. Store the width of the primes in a box, so we can get its width
    \setbox\scratchbox=\hbox{$#upper$}%
    % 2. We'll want to kern back by a bit more than
    \scratchdimen=\dimexpr \wd\scratchbox + 0.3em \relax%
    % The actual typesetting
    \kern 0em #upper\kern -\scratchdimen}

\starttext
    % Quick and dirty 1: just pretend the prime is uppercased
    $f'_x$, $f_x'$ $f^{\prime\prime}_x$.

    % Less quick, still dirty: the \myprime macro defined above
    $f'_x$, $f_x'$ $f\myprime{\prime\prime}_x$.

    % Slow and dirty: add a kern between the \primes,
    % because bonum pushes them apart a bit.
    $f'_x$, $f_x'$ $f\myprime{\prime\kern -0.1em \prime}_x$.
\stoptext

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