Summary.

-ого and -его are pronounced with "в" (v) sound ONLY when they are SINGULAR GENITIVE endings.
(The same is true for genitive pronouns: его, него, кого, того, сего, чего etc.).

In all other cases, the regular "г" (g) is pronounced. Compare:
Я узнал много [mnO-ga] нового [nO-va-va]. - I learned a lot of new (information). Here "много" (many, much, a lot) does not have any genitive ending, so it is [g]. But "нового" is the genitive form of "новый" (new), so you pronounce "г" as [v].
100 долларов? - Это дорого! [dO-ra-ga]. 100 dollars? - It is expensive! Here, "дорого" is an adverbal form of "дорогой" (expensive), there is no genitive ending.
Я купил 2 бутылки дорогого [da-ra-gO-va] вина. - I bought 2 bottles of expensive wine. The first "г" belongs to the root, it is [g], the second "г" belongs to the genitive ending -ого, it is [v].