Do I really have to use the yarn that pattern calls for?

One of the readers commented in "Diagonal Lace Baktus" post that it's good to have a pattern for an affordable yarn.

You know,  if fact, MANY-MANY-MANY pattern that are written for a pricier yarn can be adopted for a cheaper one. In many cases, the price of yarn or even  a fiber content will not be a defining factor in how the final product looks like.
Here is an example for you:
I have designed this "FUN TWIRLS HAT" for KnitPicks' Interdependent designer program. The hat is knitted in cotton yarn (well 75% cotton/25% acrylic)

Now,  the "trial hat" was knitted in 100% acrylic yarn bought in local JoAnn's store. Here it is, presented by the same model:

Do you really see that difference in fiber content? Nah...  the yarn you choose depends on your preferences.
I personally can not stand even a little itch in my sweaters.  Neither does anyone in my family. On the other hand cotton yarns feel a little too heavy to me.

I am most comfortable in nice comfy acrylic ones. So I adopt the patterns.

How to adopt the pattern for a different yarn?
1. First and foremost: look for the same yardage. It should be about the same yards/oz (or meters/g).
Some manufactures give you a little  help: they give a yarn a number to signify the weight of yarn, which is essentially yards/oz.

Some manufactures don't do that  - so your best bet is that yards/oz value.
Just a reminder to use a little math: 100 yards/3 oz is the same as 200 yards /6 oz .

2. Look for similar texture: if the original pattern calls for a smooth yarn - use somewhat smooth yarn. Obvious? Well, I recall the case when a person tried to knit this stole (found here) from black boucle yarn. Not only it was a frustrating knitting time, the result was far from desired: the lace pattern was lost in texture of the yarn.

Hope that was helpful. 
Happy knitting everyone!

P.S. Well, this blog is about knitted lace. So here is your warning: it is a little trickier to do yarn substitutions in lace patterns that require pin-blocking. I think I will write a separate blog post about that.