I guess really its down to what you think is a fair price, I know once I have the logins I need it will take me less than 5 minutes to complete the job(assuming their server is not overloaded and slow), so to me charging much more would seem unfair, after all happy customers are repeat customers. Perhaps from their view having WP installed for them would be worth $50/$75/more so its down to what you judge to be a price they will accept if you want to push the most profit you can possibly get.
I did once have a potential client reject me to go with someone else who quoted them $50 for the exact same thing, they stated to me they would not use me because they thought for the price I would probably not do a 'good job'.
Clearly they knew very little about whats actually involved and good luck to people who want to charge that much for a tiny effort. I feel I charge a very fair price but maybe I should put it up, I might get more business
