Why metal to gold?
1. That is the definition of Alchemy
2. can Be a skill that allows for economic growth
3. will promote Character trades, RP , Alt High Value currency, black market,
4. another use for existing metals ( adding new value to them ) , and perhaps as well existing herbs
5. a new trade path that brings together mining, metal refining ( metalurgy ), and Alchemy
6. alchemy can be both magic spell and apparatus based.... or either.
 
beginer Alchemist  needs several refined metals combined to form one gold bar... i suggest refined  iron, coal, tin, copper in multiples, resulting in over all cost ( value of the refiend metals used ) to exceed the value of the refined gold bar.
    failures destroy all elements used
Low level Alchemist   needs fewer of each metal used, with more frequent successes

Mid level Alchemist   Fewer still and more frequent success

High level   one of each metal  with frequent successes

  and then ... like the usual.... potions can be made from herbs