It really depends on your area and market. I wouldn't be able to advise you of the exact pricing but I can give you some tools to use.
In garment creator, there is a cost estimate feature, it's pretty accurate. At the very least you can now work out how much each shirt costs.
If you are using a RIP, most also have a cost estimate feature. If not, you can always print out how much ink was used after the shirt is printed to calculate it yourself.
Once you get the cost, you would want to consider whether you want to use the average cost for a full front or pocket and charge everyone the same or charge by a case to case basis. Ink cost won't make a huge difference for pockets but for full front shirts, there could be a significant difference in ink cost.
Also keep in mind that a white shirt is significantly cheaper to produce as it does not require pretreatment or white ink. So, you might want to look at the pricing between a white and dark shirt, do you want to leave a big pricing difference between white and dark shirts or do you want to raise the price of white shirts to make dark shirts more competitive?
Then you should consider your competition. Such as other DTG printers and mainly screen printers. What are they charging? Then look at your own value proposition. Are you going to be the shop that offers the cheapest price or the shop that offers additional services such as fast turnaround, graphics consultation etc., to justify the higher price per shirt.