Art values are determined not by the artists but by official art appraisers (who cahrge $ for their services)....
Someone can buy a young artists work, watch it go up in value, have it appraised at current high value , donate it and get a nice write off , the artist gets zilch.
Meanwhile should said artist die , the estate taxes on that work is valued at appraised value.
Well, then the artist should wait before selling. Way too many of self-proclaimed Michelangelos for my liking. IMHO, their crap [>99% of their output] is justly - nay, generously - valued at the cost of materials. As for those appraisers - I wouldn't be buying an used car from any of them.