Strictly speaking, the "marginal cost" is the cost to produce "one more". For complicated functions, that is difficult to calculate and can be approximated by the derivative- and depends upon how many are being produced.
For a linear function, however, that is easy. If c(x)= 20.4 - 0.0007x, then c(x+ 1)= 20.4- 0.0007(x+ 1)= 20.4- 0.007x- 0.0007. Here, the cost of producing x+ 1 is -0.007, 0.0007 less than producing x. The marginal cost is -0.0007 no matter what x is.
Or, in terms of the derivative, yes, the derivative is c'(x)= -0.0007, a constant function. But that just means that c(1255)= -0.0007- the marginal cost is -0.0007.
You might want to check your problem again- costing less to produce more is a bit unusual!