Off hand, I would say just giving an extra 1% metal per wave completion would be enough. So you'd get 200, then 202, then 204, and so on until 220, then it jumps to 223. If you follow this out, by the time you're past 50 rounds at 1% a round, you're earning 328 metal upon completion. It's not much, but it can make a difference and it's very simple to change. It could even start at a higher percentage but decreases every so many rounds until it's only giving 1%.
As a means of balancing, there are a lot of ways this could be done. However, one particular idea hit me... why not make it each turret has multiple effects you can upgrade for lower metal prices? So you can upgrade damage, attack range, attack speed, etc.? That way, you can then add more things for each turret, some of which could be unique. There are a few ideas I can name off, but I'm gonna hold onto them for now so you have more room to flex your thinking. :D