A paid portal can be metaphorically equivalent to going to a friend's house. The more you go to his house, the more you get acquainted to stuff and people there.
If the entrance keeps changing every time you visit, it will be difficult to find the house itself. If the kitchen and drawing room are swapped, you are likely to get disoriented.
Similarly, there are a few key pointers to make sure that the user experience of the portal is not hampered while placing the money making banner.
the top band usually holds the branding of any portal which assures the user of the logged in zone.
a. If the visual weight of the logo is really good, then we could place a simple text based ad without much jarring visual near it.
b. If the visual weight of the logo needs emphasis then the banner could be timed to appear post say [n] sec after the user has logged in.
The placement of these ads should be such that the user gets introduced to them but at the same time know which section to avoid looking at while using rest of the portal. Remember, your users have paid money for a purpose and they would definitely want to feel the worth.
Also, the visuals of the banners should be minimal so that it is not hindering the readability and consumption of rest of the portal.