What goes into the price of custom merch: quantity, ink colors, print locations, garments and turnaround.
The blank sets the floor for your price. With the same 1-color front print on 200 shirts, a Gildan 5000 is $6.18 a shirt, a Bella Canvas 3001 $7.96 and an AS Colour 5001 $10.48. A cheaper blank isn't wrong, it's a different shirt for a different customer. Pick the blank first.
Less than most people think. On 200 Bella Canvas 3001 tees with a front print, one color is $7.96 a shirt and six colors $9.46, about 30¢ a color. It's more on smaller runs (45¢ at 100 shirts, 75¢ at 50). Threadbird charges no screen fees, including the white underbase on colored shirts.
Much more than an extra color, because each location is its own run on the press. On 200 Bella Canvas 3001 tees with 1 color per location, front only is $7.96 a shirt, front and back $10.01, and front, back and sleeve $12.06. That's about $2.05 a location, against 30¢ for a color.
Order what you can sell, not what gets the lowest price per shirt. A 1-color front on a Bella Canvas 3001 is $15.46 a shirt at 24 and $7.06 at 500, because setup costs the same at any quantity. But 500 shirts is a $3,530 order. If you're reselling, 100 to 200 is usually the right range.
Start from your retail price. Rule of thumb: retail at least three times your landed cost (the shirt plus shipping, tags and packaging), so a $35 shirt should land under about $11.67. At 200 shirts, an AS Colour 5001 front print is $10.48, a Gildan 5000 front, back and sleeve $10.28, and a Bella Canvas 3001 front and back $10.01.