Substituting Yarns?

Most of the time I don't really want to use the yarn shown in knitting patterns. For instance, in the latest issue of Vogue Knitting, the cabled sweater on page 74 would cost $450 to knit the smallest size in the yarn called for!

Can you offer any guidance on substituting yarns in a pattern? Whether I have a specific yarn in mind, or I want to find a different yarn.

Yardage/ weight/ needle size/ gauge

Most magazine patterns are written to sell yarns. Luckily, many of today's best knitrags have a yarn weight guide to take care of the hardest part of substitution (ie.. Knitter's has the 1-5 number system)
If not, some will at least tell you the yardage/weight of yarn used (ie..sport @ 135 yds/ 50 gr, bulky @ 65yds/ 50 gr)
By the same token, most yarns have a recommended needle size and gauge on the label. If those numbers on the yarn you are wanting to substitute match the pattern, you're set. If not, ask me! : ) There is a nice little proportion to use if the gauges don't match, but I always suggest swatching first to check YOUR gauge with the sub yarn.
There are several other considerations, of course .... fiber content, hand, drape, coloration but once you know the weight yarn called for, the rest is more personal taste.