I can only give a theory as my advice:

Normal stuff that you can easily buy from shopkeepers: price x 1,5 = street price.

Drop stuff from mobs: rank x 30 = street price

Compound stuff without alchemy book:
- equipments + weapons: level x 1000 = street price
- materials: rank x 200

Compound stuff with alchemy book:
- equipments + weapons: level x 1500 = street price
But check first if that weapon or equipment has an alchemy book in the recipe.

But you can make it more expensive or cheaper, if you want.