The cost of any equipment you buy, space/wiring you need to modify, etc, gets spread across the number of shirts you use it on. If you end up buying a $20,000 DTG printer or a $5,000 screen exposure unit, $2,500 screen press, and $4,000 conveyor tunnel ... well, you'll have to print a few shirts to justify that expense.
With patience (and some luck), one can find a good deal on used equipment. Or, if you are handy, you might be able to build some of the equipment for less money, but more time.
Weigh how much time, patience, DIY skills, and money you have access to and how much of each you are willing to risk.
If you are just starting and are uncertain how well your designs will sell or how well you will like doing all of this, then it may well be smart to outsource production. You can always take over the printing yourself once you've sorted out the rest of the business.
A sort of in between option is to outsource the printing of your art as Plastisol heat transfers, and then heat press them onto the shirts yourself. That gets you involved in the production process but greatly limits the cost and space required for equipment. This also gives you extra flexibility in terms of inventory, as you can press a design onto whatever size, color, and style of shirt that you want, as opposed to being stuck with 100 extra-small pink V-necks imprinted with an image of a screaming duck giving the finger, while you are totally sold out of that same design in large black Ts.
As to the bigger picture of which way has the bigger payoff in the long run ... well, people will argue hard in both directions. My opinion is it matters more what you like to do. If you enjoy printing shirts, then you might as well pay yourself to do it rather than pay someone else to do it. If your business grows to the point that there isn't time for you to do it all yourself, then you need to decide where your presence makes the most difference and what tasks you are better off paying someone else to do. In short, if you are really small, or really big, outsourcing the printing can make a lot of sense. If you are in between those points, it is a matter of whether you like to do it or not, and what your future plans are in terms of size. Myself, I have no desire to get larger than what I can do on my own (and would raise prices to reduce demand if it got too busy... not that I expect to have that "problem!").