Sheets at garage sales, thrift stores, or salvage stores can be a good source of cheap fabric, useful for everything from quilt backing (scrap quilts you plan to use, anyway, maybe not art quilts) and curtains to clothing. A young man I once met kept a photo of his cute and very nicely-dressed little girl on his desk. After bragging on the daughter, he said that his mother made the dress in the photo, as well as a lot of other clothes the girl wore. "She buys floral sheets on sale and makes these dresses, and they look like Laura Ashley dresses you'd pay a lot of money for in a store." He was right, it did look like a Laura Ashley dress. A lot of it probably depended on his mother's sewing skill, but it shows you can get good results from sheets.
I haven't done this much though. Mainly because I'm picky and seldom find ones I like, but there have also been times I wondered if a particular thrift store sheet was a good buy or not. To make that determination easier, I've figured out approximate yardages for the most common sheet sizes.
To do this I looked up a chart of average sheet sizes. Any particular manufacter may use slightly different measurements, so you have to check the tag to be sure of the measurements you're getting, but as secondhand sheets may have faded or absent tags, I thought figuring the averages would give me a helpful rule of thumb to go by on price, as well as letting me know whether a particular sheet has enough yardage for what I want.
Note: The chart I used said that its measurements for fitted sheets included top surface only, so I'm not including sides in my estimates. I haven't tried it, but I'm guessing that if you snip off the elastic and press the sides flat, you may add 6" around the edges. Depending on what you're using it for, you may lose some fabric around the edges of flat sheets, because of the hems; OTOH, if they're secondhand, the flat sheets are apt to be in better shape.
Fitted--just over 2 yds of 39" wide fabric.
Flat--2&2/3 yds of 66" wide fabric.
Fitted--just over 2 yds of 54" wide fabric.
Flat--2&2/3 yds of 81" wide fabric.
Fitted--almost 2&1/4 yds of 60" wide fabric.
Flat--2 & 5/6 yds (i.e. just over 2&3/4) of 90" fabric. If you were to cut the fabric up the middle, you'd have two 2&5/6 yd lengths of 45" wide fabric, for a total of 5& 2/3 yds of 45" fabric.
Fitted--almost 2&1/4 yds of 76"-78" wide fabric.
Flat--2&5/6 yds (i.e. just over 2&3/4) of 108" wide fabric. Cut up the middle, that yields two 2&5/6 yd lengths of 54" wide fabric, for a total of 5&2/3 yds of 54" fabric. OR you could turn the sheet side ways and have 3 yds of 102" wide fabric; cut that up the middle and you have two 3 yd lengths of 51" wide fabric, for a total of 6 yds.
The rule of thumb: Any sheet twin-size or larger is going to give you at least two yards of fabric (although on the twin fitted it will be a narrower width than most contemporary patterns call for in their pattern layouts), so take the price of the sheet, halve it, and ask yourself if you'd pay that much per yard for that fabric. If it's close, remember that you're actually paying a bit less per yard, especially on a queen or king flat sheet.