The general fee
Applying for naturalization with Form N-400 costs $710 filed online and $760 on paper. That is the figure you will find nearly everywhere, and in this case the sources actually agree.
The two tiers almost nobody publishes
Reduced fee of $380. If your documented household income is at or below 400% of the Federal Poverty Guidelines, you pay half. This is not an extreme-poverty exemption: 400% is a fairly high ceiling, and many people who qualify do not know it. For a single-person household, that threshold sits around $63,840 a year.
Full waiver. Applicants who meet the requirements of INA sections 328 or 329 through military service pay nothing.
The online filing trap
Here is a detail that costs money if you miss it: the reduced fee and the waiver can only be requested on paper. Neither exists as an option in the online filing flow.
So if you qualify for $380 and file online to save the $50 difference, you end up paying $710 instead of $380. The online discount is only real if you are paying the general fee.
Watch for changes
There are Department of Homeland Security proposals to raise the general N-400 fee. None is in effect as of this page's verification date, but it is exactly the kind of figure worth re-checking before you file. The verification date is at the bottom.
What is not included
Photos, if taken at a studio; attorney or accredited representative fees; and the cost of obtaining documents or translations. The N-400 fee is the only thing USCIS charges.