In this case de is a preposition used to complement the noun prix. It is not partitive article used to indicate a certain amount. In your terms indeed it is simply “de” (which translates to of) followed by “la salade de tomates”.

What happens in the question you linked is quite different because the complement does not come right next to the noun but is provided as an attribute after the verb être. Moreover “Le prix est de” can only be used to indicate the amount not the object whose price is mentioned.