Kimsie,
I don't think you can truly compare your prices and a screen printers prices because really, you both cater to different markets, though there is some crossover in the market. A screenprinter generally will not print one shirt for a customer, but may take an order of 12. A DTG generally will not take an order of 1000 shirts, but may take an order of even 200 or so. That will depend on the DTG operator's feelings on profit vs. investment.
Since your original question uses screen printing pricing for comparison, to really find out how you compare to a screen printer's prices for the same order, why not call around to some of your local screen printers with the above order, and find out if anyone will print you 1 to 3 shirts, and what the charge would be... then you will know what the price is that your customers will hear before they come to see you.
This is where you and your DTG machine will shine. My guess is you will hear that they won't print that low of a quantity, or the amount will be very high. My guess is you will find that you do not need to go to the lowest price rung to compete.
Also, if you take the above screen print numbers and use the lowest ones:
$15 screen charge plus $6 for a shirt, the total cost for one shirt (if they will even do one shirt at this price) is $21. Why would you charge $8? You could charge $18.00 and beat the screen printed by almost a full 15% discount off the price.