Learn Russian: По-моему, твои расценки зависят от портфолио. - In my opinion, your rates depend on the portfolio.

You could use it to introduce the topic at the start, for example: Насчёт твоих расценок, по-моему, они зависят от портфолио (Regarding your rates, in my opinion, they depend on the portfolio). However, you cannot replace от with насчёт after the verb зависеть. That verb strictly requires от + Genitive.

Why is the preposition от used here instead of на?

In English we say 'depend on', but the Russian verb зависеть always pairs with от (literally 'depend from') followed by the Genitive case. This is a fixed verb-preposition combination that B2 learners should have fully memorized.

Since от requires the Genitive case, why doesn't the ending of портфолио change?

Портфолио is an indeclinable borrowed noun. Just like words such as метро, кино, or такси, it keeps the exact same form across all grammatical cases and numbers.

Is the comma after По-моему mandatory?

Yes. По-моему (in my opinion) acts as an introductory word here. In Russian punctuation rules, introductory words expressing the speaker's stance or the source of information must be separated from the rest of the sentence by a comma.