This afghan (which was made by my husbands great aunt for her sister in the early part of the last century) is a perfect example of a project that can only be considered an absolute labour of LOVE as it is knit entirely of garter stitch sections in FINE, 4 ply fingering weight yarn. When it comes to placing a value on reproducing such a knitted project, it is without price (as most of us would die of boredom long before it was done).
We have so many amazing knitters that we meet each day, and also many who are extremely competent yet all of these craftspeople and artisans often feel shy about answering this rather sensible question asked by those who don't know about such things: "What would you charge to knit....(whatever) for me?"
Despite the fact that anyone who would even consider taking on a knitting or crochet project for money must absolutely LOVE knitting, and be fairly enamoured with the concept of a bit of a challenge in their knitting, it shouldn't take away from the reality that you are giving up a substantial number of hours of your skilled work to produce something for someone who absolutely must realize how valueable that work is.
The people who are asking you to knit something for them usually fall into one of 2 categories: the first might remember the beauty of the handknit article that they received in their past and recognize how rare it is to have someone care enough for you to produce such a garment and are willing to pay accordingly. The second group is associating your work/ability and the finished project with the pair of mitts made of sayelle left over from 1973 that they saw at the church bazaar last fall for $3./pair.
My advice is don't even consider the project unless the person is willing to accept the fact that it requires good quality yarn. You deserve to enjoy the feel of what you're doing. Then when it comes time to charge for your labour, you should at least receive the equivalent price of what was paid for that good quality yarn. (I can almost guarantee that it won't net you much more than $1/hour but at least it's something.) If the pattern is in anyway complicated, altered, hard to understand, boring beyond belief or inordinately time consuming then you should charge twice what the good quality yarn is worth for your part in the process.