As Mark mentioned, there are extras, and that depends on the country/state you're visiting. The price quoted, much like airlines up until a few years ago, usually excludes insurance and/or local taxes.
If you get through to the payment page (without actually paying) on one of the popular rental company website you'll have a fully detailed invoice to look at which will include all local additional taxes and any specifically mandatory insurance for the location. Once you have those percentage/daily fee details you have an idea of any additions any prospective comparison site may be missing.
The best way to guarantee your budget is to prepay by credit card if that is an option. They'll always try to up-sell when you're getting the keys, and you can happily say no to every option, especially if you work out where the closest petrol station is to where you're renting from and if possible work out the litre/gallon cost there, then you can calculate whether paying for the 'tank of gas' is worth it.