When building small, light models, it is important to use light materials. A few grams here and there add up to ounces. For example, when building Tiny, I started comparing the weight of various components and found that by selecting good materials and parts, I could avoid about one ounce of weight difference.
I use three different scales for weighing models and parts. The first
is the My Weigh 1 Scale 2000 digital kitchen scale from Rollies.com for
under $50. It weighs in units of grams or four-hundreths (.04) of an
ounce (Rollies does not indicate that it will weigh in ounces - it
does.)
The other two scales are pocket pen-scales, shown to the right of the kitchen scale in the photo above. One weighs small items up to 10 grams in .1 gram units and the other weighs medium items up to 100 grams in 1 gram units. The 100 gram scale, in the photo to the right, is the one that I take to the hobby shop to weigh balsa sheets. The pen scales are under $10 each, also from Rollies.com.
When I purchase balsa sheets, I write the sheet weight on the label or in a corner. I then use the sheet from the other end. Going through my balsa scrap box, I know the weight of each full sheet and whether it is appropriate for a specific project. (Rollies also has picures of each scale on their web site.)
Another place that people have found inexpensive, and accurate scales is www.oldwillknott.com. Others have reported buying good scales in the $30 range from Ebay.