Releasing a new brand is not about profit, it's about brand recognition and building a customer base.
Don't worry about making money at first. Just sell shirts. If your price point is $25 a shirt, starting out means selling shirts, even if you have to pay $22 a piece to get them made.
Once you have built a customer base, you can then start looking for the least expensive solution.
What kills most brands dead is a lack of movement. You need great service, great quality prints and new designs regularly. Movement leads to momentum leads to brand recognition leads to volume.
And until you have the volume, profit will be minimal.
My #1 brand needs to move 66 shirts a day in order for me to earn $100,000 a year profit. The first 3 or 4 months for me meant LOSING thousands of dollars a month in giveaways, contests, and advertising. I didn't care because I was building a brand.
I meet people every day who want to start a garment line. I ask them: do you have thousands of dollars in savings to build your brand name? I spent about $10,000 out of pocket to do it. $0 of that was for hardware, ink, cotton blanks. All of it was for getting the word out.
It is a hypercompetitive market. 500 new brands come into existence every week. 495 of them fail. You can spend 100-200 hours making 5-10 new designs. All 10 of them will be old news in 90 days. That means spending 1-2 hours a day, every day, making new designs. Another 1-2 hours a day advertising and marketing. Another 10-15 hours a week meeting your customers where they go. Another 10-15 hours a week communicating with old customers to ask for comments and criticism.
But, there is a lot of money to be made. Like in any business, going cheap is going to kill your brand.