I bought and sold my 75 for $275. A dollar per gallon is great for just a tank, but if you think you can find a complete 75 gallon setup for $75 you're dreaming.
Whether the tank is 3 months old or 3 years old doesn't matter - used is used. Hopefully your friend is realistic and doesn't try to recoup the money they spent. Check out Craigslist and see what comparable tanks are being listed at, and go from there.
The fish are completely irrelevant to the price.
Were it me I would offer $150 and go from there. $100 is really low balling IMO
, which, depending on your friend, might not be received well. But then I gave a complete 55 setup to a friend for free, so I would happily take $100 from a friend for a 75. Really depends on your friend I think. In any event, know what similar used setups are going for so you can be prepared to offer a legitimate justification for your offer. However, prices vary by region so $100 might be a good offer.