The balance is not in the relative costs, it is in the fact that a shipyard is built with planetary construction factories, but expanded only by its own innate construction capability which is much slower even if much cheaper.
That depends on the situation, you play in a conventional start where you have only 500 BP/year available, then a shipyard is not faster at all, it takes almost 5 years to complete, while adding a slipway can be done in exactly 1 year.
A conventional start is balanced around converting your starting CI into TN industry before you spend a lot of resources expanding your shipyards, using your initial yards to start your expansion (which is enough to launch a survey ship from the naval yard plus colony/cargo ships from the commercial yard). TN construction factories are 10x as efficient as CI so you can get a shipyard into orbit in a year or two if you want to once you've done the conversions.
That being said, I did make an error. Shipyards are not especially slower than planetary factories; they do produce much less "BP" than planetary industry but this is balanced out by the lower cost of expanding the yard compared to building a fresh one planetside. Here I misspoke as I meant to address the difference in BP production which is not the most useful measurement anyways.
That aside, the substance of my point is the same, they are two different kinds of costs paid by two different kinds of build mechanics. Expanding shipyards is cheaper and preferable in a vacuum. Building new yards is necessary to build more ship classes in a useful amount of time, but is and should be more expensive so that it is not greatly preferable to adding new slipways to existing yards. I think in terms of the gameplay it is very reasonable, in practice I tend to find that working my shipyards at full expansion capacity makes the expense comparable to planetary industry and actual shipbuilding so I would not want to upset that balance very much in either direction.
...otherwise if the costs were equal...
Did you read the post your responding to?
No, I just happened to type a post that coincidentally was on the same topic in a freak accident of probability.