I've just knitted a man's jumper in Woolyknit Diggle which is a mid-range British wool priced at £3.50/50g. The garment took 12 balls so cost £42.00. It could have been made in a cheaper acrylic for around £22.00 but cost isn't the whole argument. The garment was for an adult (my husband) who will hand wash it (because he knows the penalty if he felts it in the washing machine!). The jumper is heavily cabled so wool will keep the structure of the cables better after washing. I well remember knitting a beautiful cabled jumper for myself in a lovely soft acrylic which looked lovely till I washed it when the yarn relaxed, the cables went flat and the jumper grew by a couple of sizes! Greg isn't likely to have a growth spurt so I don't have to worry about him growing out of an expensive knit and he doesn't find wool itchy. I think that's most of the objections to wool dealt with so now it is just cost to deal with.
I finished the jumper two weeks ago and it has been worn for at least ten out of those fourteen days. This brings me to cost per wear. Cost (£42.00) divided by number of wears (10) = £4.20/wear. Every time Greg puts the jumper on the cost per wear reduces. Wool doesn't wear out quickly and barring a washing 'incident' and based on other garments I've knitted for him, Greg will still be wearing this one in 20 years time by which time costs/wear will be measured in fractions of pence not pounds. Knitted in a cheaper yarn the jumper would lose its shape and structure after a few washes so cost per wear would be much higher. In the long term, the more expensive option is a better deal.
I know not everyone likes natural fibres finding them itchy or ethically problematic, but whatever you buy, you are going to spend a lot of time producing a handmade article so whether the yarn is natural or man-made, it's worthwhile making it in the best you can afford.