The day rate is the smallest part. Mileage caps, the security deposit hold, Salik, and the fuel policy are what move the final number.
The day rate is the number everyone compares, and it is the one that varies least between companies. What actually moves the final invoice is the mileage allowance, the deposit hold, and the three things billed at cost: Salik, fuel and any fines.
Standard hires include 250 km per day. Kilometres beyond that are charged at a per-kilometre rate stated on your confirmation. On a multi-day hire the allowance pools, so a quiet day covers a long one.
Where the number actually moves
Mileage is the most common surprise, and it is usually geography rather than driving style. Dubai to Abu Dhabi and back is roughly 280 km before you have driven anywhere at either end — over a single day's allowance on its own. If a trip like that is in the plan, say so when you book and it can be priced in rather than billed after.
Fuel is like-for-like: the car arrives full and should come back full. Salik crossings are passed on at the rate the RTA charges, with no mark-up. Longer hires are materially cheaper per day than daily rates, so a five-day booking is often worth quoting as a week.
The day rate is the part that is easy to compare. The mileage allowance and the deposit are the parts worth asking about.