Seaming can take on many colors!
Such as: how many pieces? five for a cardigan? 4 for a pullover? some other type of construction such as top down?
Anyway, I think I'd charge at least $30 for the blocking, and $30 for the seaming (mattress stitch of course)...even if it was a cardigan where you have to seam the shoulders, the sides, the sleeves seam, and the chore of seaming the sleeves into the armholes...$60 total seems about right. And I don't care if the yarn cost her only $12. The yarn could have cost another knitter $150. It isn't the price ratio of yarn cost-to-labor cost. It's the time factor.
If the pieces were already blocked, then I might charge $50 for seaming alone.
I would decline to work the finishing work such as button bands and neckline.
I'd hand the seamed piece back to the knitter, because gauge can vary from knitter to knitter.
Remember, whatever price you set for your labor sets a precedent. You can always lower your price for subsequent work, but people balk when you raise prices if you undervalued your time and labor in the first place.