We made it through Lost Mines and I allowed the players to harvest some green dragon scales from Venomfang. Two players were able to grab scales from the young dragon. I had previously allowed one player to hire some artificers to make some armor from the materials for their goblin companion for about 1300 gp. The armor was scale mail and granted the goblin resistance to poison damage. I didn't charge much because we were headed into Tomb of Annihilation. The goblin was scheduled to be taken by the death curse and was later buried in said armor. 

I've got a half-orc player that has asked to buy some breastplate and infuse? it somehow with the dragon scales (he collected 9) to gain poison resistance. 

The player currently has a +1 breastplate found in Lost Mines Dragonguard and also grants advantage on saving throws against dragon breath weapons. This player also has a +2 shield that I definitely undercharged. Current AC is 21. 

• What should I charge for this?

+1 breastplate is rare so that's a 500-5000 gp purchase outright. Then he'd have whatever costs from the armor/blacksmith for enchanting the item and putting the work into it. Dragon scale mail is very rare which bumps that up significantly. 

• Do you think he could do this with breastplate or would you bump it to scale mail?
• What advantages/abilities should the armor confer? 

my DM asked me to make a special set of armor and metal for each chromatic dragon. Noxious Orichalcum would grant immunity to poision damage, grant amphibiousness and allow you to cover a large area in poision gas 1/long rest. you could roll back those benefits. as for cost, 10,000?, 9,681? something a lot higher than 5000