To estimate the price of the 2018 Mazda 6 I4 2.5L S (Base) with 85500 km mileage, I first looked at the selling prices of similar cars. The closest match is a 2017 Mazda 6 I4 2.5L S (Base) with 155000 km sold for 46000 SAR on 2023-10-09. Given the lower mileage of the target car, it should be valued higher. Another 2016 Mazda 6 I4 2.5L S (Base) with 222000 km sold for 28000 SAR on 2024-06-30, indicating a significant depreciation with higher mileage. The listed prices for 2018 models with similar mileage are 54000 SAR and 40000 SAR, but these are not actual selling prices. Considering the selling prices and the listed prices, I estimate the market price to be slightly lower than the highest listed price but higher than the 2017 model with higher mileage.