It depends of how well you want the job done.
To do it right requires the engine bay, door jams, and inside the trunk to be painted too.
A local shop quoted me 700 dollars to repaint my 90 Miata its stock color (red). Silver could be more at the same shop for two reasons, cost of paint and because it is a color change. I know reds are not cheap, but neither is silver, while gray maybe.
Now, I might suggest that you just go find a silver Miata as the Miata will become collectible (in many ways they already are) and a color change will detract from its value.