The attack power of a martial artist is mainly determined by three factors:
Inner Strength, techniques, and weapons.
Its similar for alchemy too.
The significance of the medicinal herbs probably equates to the inner strength, whilst the alchemist's 'Medicine' level can be seen as the techniques, and the pill furnace is the weapon used for alchemy!
Just as weapons can be good or bad, so can the quality of the pill furnace.
However, regardless of the grade of the furnace, Hu Qingniu, being one of the three divine doctors in 'Eternal Chivalry' and supported by the powerful Ming Sect, his furnace's quality could not possibly be subpar.
Due to the goodwill he had accumulated before, Hu Qingniu readily agreed to Ye Weiming's request to borrow the pill furnace.
However, he did charge a rental fee of 100 gold.
This was not out of greed but following the 'Heavenly Dao's' will, it wasn't possible to let Ye Weiming have this advantage just because of his goodwill.