WOW! I started looking into the cost/environmental impact of producing textiles, and all I can say is I am absolutely dumbfounded! I looked at both cotton and polyester. In this blog post, I will look at the resources required to produce 1kg of cotton fiber and 1kg of polyester fiber. :

Cotton:

Depending on the growing area 1-3 bales of cotton can be produced per acre of land, a bale weighs approximately 500lbs. One bale of cotton can make around 1200 men's t-shirts (you can see more bale to item conversions here). Cotton is harvested with modern cotton-picking machines, these machines use rotating spindle to harvest cotton and then pass it along via a conveying system to a second machine that then removes the open bolls from the plant and finally the third machine receives the separated seed cotton and stores it until it is transferred to a separate storage container or vehicle.

Growing cotton uses a LOT of water. 20,000 liters (approximately 5300 gallons) or more is used to produce (from seed) 1kg of cotton fiber as well as about 450g of fertilizers, 16g of pesticides, and approx 60MJ (16.67kWh approx.) of energy to create. You also have to consider the fuel used sowing and harvesting the cotton as well as transporting the cotton to a factory and the kWh used to process and spin the cotton into fibers. You then have to take the fiber and manufacture a finished textile which will likely involve more water (dyeing) and electricity in several steps of the process (automated cutting, automated sewing, conveyance between these processes, etc.) and the shipping costs to get the good from a factory -> shipper -> warehouse -> store -> you. Just creating 1kg of cotton fiber generates 10-15kg of carbon dioxide emissions.

Polyester:

Polyester requires far fewer resources but is still mind-boggling. Polyester is synthetic, and it is produced from fossil fuels. Forgetting the necessary amount of energy to get oil out of the ground and refine it for use: 1kg of polyester requires approximately 1.5kg of oil, 17 liters of water, and just shy of 100MJ (27.75kWh approx.) of energy to create. Creating 1kg of polyester generates approximately 2.3kg of carbon dioxide emissions.

Guys! This is insane! I love cotton clothes, but WOW I'm going to focus on mostly synthetic fiber clothing going forward, and even then, the resource usage is just mind-boggling to produce a synthetic textile garment! I like 100% cotton because when a garment is no longer wearable, I could cut it up and compost it. Still, my research shows me most cotton-growing operations use absurd amounts of irrigation, which is just disruptive to the local environment as well as wherever the water is being trucked or piped in from.

Some of my sources:

http://wwf.panda.org/about_our_earth/about_freshwater/freshwater_problems/thirsty_crops/cotton/

http://www.sustainability-ed.org.uk/pages/example4-3.htm

http://cottonaustralia.com.au/cotton-library/fact-sheets/cotton-fact-file-water

Update, September 4th 2017:

So I got a little curious and wanted to try and get an idea of what 1 acre of cotton means in yield, also what sort of global power demand producing cotton fiber from the ground up looks like.

There are something like 167 million acres of cotton planted worldwide, that's more than 261,000 square miles of cotton. It looks like 29 million tons of cotton are produced a year right now, that's 133,195,950 bales. That's roughly 1.25 bales per acre, so 272kg pounds of cotton per acre. A quick google search shows 6-9.5 ounces per t-shirt, giving you an estimate of 1238 t-shirts per acre.

So if we look at averages, you need a season and an acre of land to make a bit over 272kg of t-shirts. For that 272kg of t-shirts, you'd also need about 1.4 million gallons of water, 122kg of fertilizers, 4.3kg of pesticides, and approximately 4534 kWh of electricity (average American household in 2015 had an electricity consumption of about 10,812 kilowatt-hours). That means the 167 million acres of cotton being grown worldwide uses about 757 TWh of electricity to make cotton fibers. Total world electricity consumption was 19,504 TWh in 2013, that's like 3.8% of the world's power consumption just to get from an open field to spun cotton fibers.

Insanity.

Update, July 3rd 2018:

There's a problem with polyester. Polyester itself is a source of pollution.

First, the manufacturing process results in a host of carcinogenic compounds being used (and in parts of the world with lax regulation, being released into the environment) as well as the fact that during the washing process, you are introducing microplastics/microfibers to the environment. One article states that as much as 85% of human-made materials found on the shoreline are microfibers like nylon and acrylic, which are ubiquitous fibers for textile production. Ouch!

While the danger of microplastics as environmental contaminants is still widely unknown, more and more studies are being conducted to see the extent of damage already done as well as future implications. One fact for sure, though, is we are creating a new geological age which some geologists wish to call the 'Anthropocene' epoch, which includes changes such as anthropogenic climate change and the introduction of synthetic materials into not just landfills but ecosystems the world over. This is concerning.

We also have the fact that 70 billion barrels of oil are used each year to manufacture just polyester! Insanity! With an ever-growing population, what are we to do? Cotton isn't a realistic answer, synthetic and most (if not all?) artificial fibers have long-term environmental ramifications due to the way they break down and linger. Sure, we have discovered that mealworms can digest some synthetic materials into environmentally safe products, but that will only help a fraction of a percent in dealing with this problem in any realistically scalable attempt.

Yet another problem we need to start thinking long and hard about.