The cheapest way is to use the hot knife blade method. Basically clip the tip off of an X-Acto knife, heat it up and use it to carefully inscribe the surface of the plastic with the zimmerit pattern. Some folks have adapted this to the tip of a soldering iron, others have had a candle burning next to them to continuously reheat the tip.
I'd practice on an old kit, the flat areas of the scrap sprue or the inner surface of the hull before I't try it on the outside.
Google hot knife blade zimmerit for a bunch of links of folks who did it.