I would have gone with cheaper, TBH. Go with a single meson turret, minimal armor, and keep it under 500 tons. You also should save by exploiting the fact that the PDC is based on a populated planet. Give it a 1 month (or less) deployment time. With a fighter fire control, (and a fighter PDC fire control at that), you should easily be able to match the turret's tracking speed.
With PDCs, if the goal is 'cheap', consider that when paying for 1 level of armor, you get 4 free. So if you want to maximize your free armor, you want a lot of level 1 armor.
Also, if you build them fighter sized, the enemy won't find it as easy to target them.
Armor costs Duranium, which is almost always a limiting factor in your economy.