The amount of un science in these posts make me sad.
Yes you can. It won't be any more expensive than a ship minted weapon assuming you're targeting ships in low orbit. Otherwise, you'd have to increase the ranges by a lot.
Actually you won't.
In fact, it'd be cheaper because most of the materials could be harvested on planet.
And since you're using laser weaponry, exit velocity is a nonissue. So is atmospheric friction. Actually that wasn't a true issue in the first place.
Targeting is the major issue, but since we're talking about existing technology that can hit a dime on a plate from a thousand kilometers, targeting isn't an issue either.
But, to answer your question, yes, you would need a submission and maybe even a Dev Thread, depending on the whims of the judge reviewing your entry.