Sewing patterns can be expensive, so I try to buy them on sale. That's not the most frugal sewing tip I have, though. This is: Find a versatile pattern that you like and use it over and over again. Of course, if you can find one that covers a large range of sizes, you will get more use out of it and more for your money.
I love this Anne pattern from Children's Corner. It is very versatile, too. I have made it with short sleeves, no sleeves, long sleeves, with a Peter Pan collar, and with a smocked collar. (See the second photo. Also, see the middle dress in my header.)Here are some examples of the Mary De pattern (the sleeveless ones) that I have used many times. I have made these particular patterns so many times, that I can do them now without looking at the directions.
Once I made the one time initial investment in the pattern, my only expense after that was the fabric for the dresses. If I find a good sale on fabric, I can make a beautiful smocked dress (or I can leave out the smocking) for just a couple of dollars!
You don't have to know how to smock or even be an expert seamstress to pick out a pattern and, with a little practice, learn to use it well. You can learn to make beautiful hand sewn garments for very little money.
Be sure to check out Biblical Womanhood for more frugal tips.