Question: evalf abs encapsulated

given the following:

param:= {mu[2] = (1600), mu[4] = (1500)}
R := unapply(1/(1+exp((1/50)*x)), x);

prior := evalf(expand(eval(eval(R(abs(mu[4]-mu[2])), `` = (x->x)), param)));
   

I need to keep the paramaters encapsulated. I would like this last step to output the answer 0.1192029.... but the abs function doesn't let me

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