Yes, sort of.
I booked a car through Kayak in Vancouver, Canada.
It was as cheap as it said, however there are other costs to remember on the ground.
For us that included: extra driver charges for a second driver insurance for the car (third party or full depending on the country)
some additional taxes might apply credit card charges might apply Kayak can only quote the price provided by the source.
For example their Easyjet and RyanAir and Jetstar flights are always a bit under, as RyanAir especially is infamous for adding lots of extra charges as you purchase your flight.
travel.stackexchange.com/users/101/Mark Mayo
Are the prices on car rental price comparison sites (eg: Kayak, Expedia, etc) what you will really end up paying?
Or are there a bunch of hidden "extras"?
No, in nearly every case, you will pay more than the Kayak etc sticker price.
But this is not because you have to, but because the price excludes most of the extras that any reasonable person hiring a car is likely to choose - the key one being complete insurance, but as Mark Mayo mentioned there are others.
I'd like to know how to more accurately plan out such expenses when planning future vacations
The surefire approach to this is to check the rental company and attempt to book the same car through them directly.
Make sure you select all the extras you need, and the price before checkout will be an accurate reflection of what you will spend.
Another good thing to do though, is check if any of your credit cards offer eg.
loss damage waiver cover or other extra car rental insurance when paying with that card
(this is a typical feature of platinum cards).
If so, then you just need to cultivate a clear understanding of what insurance is included in the 'no-frills' product offered by Kayak et al, and whether your credit card insurance closes the gap sufficiently for your comfort/risk appetite.
Then you can keep on booking with the convenience of Kayak, but know you have the cover you need.
Just make sure you read the fine-print on the credit card provided insurance, because as this Wiki article details, there are often many exceptions or limitations.
travel.stackexchange.com/users/4272/imoatama
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.
travel.stackexchange.com/users/2000/dsample
I think sometimes for some rental companies comparison sites are right on the dot.
Other times they estimate the costs without the little (or not so little) "extras" rental agencies add once you get there.
However, I tend to like to do a bit of research first on local car rental companies and try to book thru their sites in the local currency (Google translate helps).
It can often be much cheaper.
Sometimes even 50% cheaper!
After I do that I them try the comparison websites to see IF the local rental car companies even show up, and how they compare.
I do similar things for airline tickets...
when possible.
For example: I recently booked a 3 day rental in Brazil with Movida (a Brazilian company) for about R$210.0 total (all insurance included) which is basically R$70/day or ~UDS$35.
Thru
comparison sites rates with other "international" rental companies were in the USD$50-65 range, if I recall correctly.
travel.stackexchange.com/users/9504/unknownprotocol
I have used holidayautos.com (from the UK) to book cars in a few countries, all in cases full insurance etc was included and there were no hidden charges.
However if you use a USA site to book a car in the USA
it is more complex as a lot of people already have their own insurance that cover hire cars, so prices are often quoted without insurance.
travel.stackexchange.com/users/1281/Ian Ringrose
Are the prices on car rental price comparison sites (eg: Kayak, Expedia, etc) what you will really end up paying? Or are there a bunch of hidden "extras" or other additional charges that you will also need to budget for when planning a car rental?   It seems like we ended up spending a lot more than expected the last time we rented a car, and I'd like to know how to more accurately plan out a budget for such expenses when planning future vacations or trips.
Are prices on car rental comparison sites the "real" price?
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travel.stackexchange.com/users/520/Jessica Brown
In most cases, the price excludes most of the extras that any reasonable person hiring a car is likely to choose - the key one being complete insurance. However, this depends on the country/state you are visiting as well as the company: holidayautos.com (from the UK) includes full insurance and has no hidden fees. In any case, the final payment page will give you a detailed invoice including all of the local additional taxes and any specifically mandatory insurance for the location. Prices for car rental on Kayak are usually not the final price as many extras are excluded. It is recommended to check prices on a car rental website to see the true cost. A UK site, holidayautos.com, quotes full prices with no hidden surprises.
Kayak does not usually display the final price for car rentals as it excludes a lot of extras that you may consider essential. You can get a better idea of the final price by doing a ghost booking on a car rental sight and see what the total comes to.  holidayautos.com is a UK site that gives the full price with no surprises.