might be a dumb question but the prices between jewlery vary highly - just found a necklace which would cost me 2500 at a trader (without any enchantments) - is it relevant for enchanting how valuable the jewlery is - therefore the higher the value, the higher the enchantment you can place on it...or does it not matter at all?

No, I tested it myself the other day. Cheap jewelery enchants just as well as expensive jewelery.