Seems times have changed when it comes to the way people are manufacturing, marketing, and selling t shirts and clothing nowadays. The more common and well known route people take is ordering a set number of shirts in hopes of getting bulk deal to lower the price per unit, but also requires up front capital which you don't get back until you liquidate. The more you order, the better the deal, but more risk in a sense since your capital is tied up.
Nowadays many sites offer "print on demand", you can sell your own designs on your site for whatever price and order the shirt from the manufacturer straight to the buyer. When taking this route, you miss out on the bulk order price cut but you never risk any of your own funds since you are accepting the buyers payment and using portion of new funds to order shirt from the manufacturer straight to buyer without ever having to package or ship the product yourself.

So how low do these prices get?

This might be a surprise to the people who have never looked this deep into the products they buy, but that same shirt you bought at the mall or online for $20-30 was probably printed for about $2-5 USD.

Bulk Orders

I'm looking for all the information and input I can find so please let me know if you have any experience with designing and ordering your own shirts. One of the best deals I have found so far on bulk orders is from "4imprint.com" Prices range from as low as $2.75 per unit when you buy 1008, and high as $7.85 per unit when ordering the minimum of 18.

So when ordering the minimum of 18 shirts at $7.85 each, your cost is $141.30
Sell each shirt for $20 for total of $360 net and $218.70 profit
Sell each shirt for $30 for total of $540 net and $398.70 profit

When ordering 36 units at $5.59 each, your cost is $201.24
Sell each shirt for $20 for total of $720 net and $518.76 profit
Sell each shirt for $30 for total of $1080 net and $878.76 profit

We are going to use 18 units for our example when comparing bulk to "made to order" since paying the $141.30 upfront cost is relatively affordable for most people looking to get into something like this, also 18 units not that hard to move.

Print On Demand (no minimum, from manufacturer straight to buyer)

Seems like pricing is pretty standard around $15 on most sites but Vistaprint.com offers custom shirts without minimum order for as low as $5 for white shirts and $10 for black and colored. Please let me know if you find better deal than VistaPrint.
You can list the shirts on your site or social media page and never have to use your own money to buy the shirts until you get money in your account from a buyer. You don't pay upfront cost but the cost of shirt is still taken and you are left with the difference as profit.
White shirt cost for 18 units is $90 and 18 black/colored shirts cost $180.
Sell 18 shirts for $20 each for total of $360 net and $270 profit on white shirts and $180 profit from black shirts.
Sell 18 shirts for $30 each for total of $540 net and $450 profit on white shirts and $360 profit from black shirts.

Bottom Line

When it comes to profit, Made to Order is more profitable when selling 18 white shirts but less profitable when selling colored since the cost is twice as much.
The profit gains become exponential when ordering larger bulk quantities since you are cutting the cost more you order.
Print on demand sites like Vista print work best with people planning to sell 20 or less and especially works well for people who don't have capital to put forth for 36 or even 18 shirts. If you are really looking for profit, bulk/wholesale is the way to go.
Please share your experience with ordering shirts, this post is purely covering cost of product, not quality so I'd like to hear if some sites or brand are better than others in terms of quality and service.
Thanks for looking.