When it comes to enchanting platinum or any metal I just tell them they can 'give' me whatever they can afford. Many are 'younger' adventurers who don't have huge bank rolls and it's good to be able to help them out. Charging obscene amounts for enchanting metal is ridiculous. Many people on veeshan will wait until I am in Freeport to get their platinum enchanted and my reputation as an honorable player in this manner has many people I haven't even met hand over hundreds of plat to me to buy them bars without a worry.

Zealott Enrapture, 40th lvl Enchantress
Officer, Seekers of Lore, Veeshan