I'm afraid it's not easy to come up with a simple price scale, given all the variables involved. But this should give you a general idea:
Are you crafty? Ever worked with handspun before? Oooh it's a whole new world. You gotta try it. I will custom dye and spin the yarn of your dreams, and darlings, I am GOOD at this. Anything from cobweb lace to super-bulky, from simple sock yarn to one-of-a-kind art yarn plied with beads and ribbons. I usually hand-dye the fiber before spinning for cool effects -- heathered, marbled, self-striping, gradient, marled, barber-pole -- but I can also dye a spun skein for that almost-solid kettle-dye effect, or hand-paint it for a variegated look.
Here's a rough estimate of the cost per 50g skein (in general, you need one of those for a hat, two for a pair of socks, six to ten for a sweater):
100% wool, wool/mohair blend, superwash wool, superwash wool/nylon blend: $10
100% merino, merino/wool blend, superwash merino, superwash merino/nylon blend: $12
wool/silk, merino/silk, more than two fibers in a blend: $14
art yarn (beads, metallic thread, ribbon, felt bobbles, etc.): $26
Notes: The wool/mohair blend is in the $10 category right now because I got an incredible deal from a supplier; it may get more expensive later. No 'blend' fiber can be done as a gradient color; the technique just doesn't work with the carder I use to blend the fibers.
The price of knitted items is in addition to the base cost of the yarn; I can make things from my handspun, or I can buy yarn to match your specs. If you want commercial yarn, your color choices might be somewhat limited, and it might end up being more expensive than the handspun. In general, cotton is cheap, wool is middling, silk is pricey.
Hat: 1 skein + $10
Scarf: 2 skeins + $20
Extra long scarf: 3 skeins + $30
Cowl: 2 skeins + $20
Arm warmers: 1 skein + $10
Mittens: 1 skein + $10
Gloves: 1 skein + $20
Socks: 2 skeins + $20
Notes: I have to charge for each skein I need to buy or make, so a multicolored pattern might be more expensive. Fair Isle, intarsia, cables, and complex pattern stitches are more work, so I'd have to charge more. With handspun or especially nice commercial yarns, I suggest we keep it simple so the yarn can strut its stuff.
Because handcrafting takes time, I'm only opening four commission slots at a go. Note me if you want one! (Or just if you have questions; I'll only put you in a slot once you're sure you want to commission something, so don't fear overcommitting. ^_^)
1. Poncho for meaikoh