The first rule is - check a few car rental offers online and only then choose one. This is a sure way of getting the cheapest offer on the best terms.

Mileage - If you rent a car for one day only, mileage is usually limited to 100 km for the day.  Starting from 3 days you have unlimited mileage.  

Cost - Price is between 25 and 50 US$ per day for the smallest car, including insurance. The prices of the local firms are not necessarily cheaper than the international ones. Advatage does seem to have a rather good daily offer, so make sure to check it out if you only want to take the car for a day or two. Budget tends to offer the cheapest options but make sure you read the fine print when it comes to insurance waivers and extra charges. 

 Car Rental Tips

• TIP 1: Always refill before giving back the car. We didn't do it, and though our tank was 65% full, we were charged for almost a full tank as the company explained that a 10% service tax was added!!!
• TIP 2: Make sure you check the drop off charge for picking up a car at the airport or returning it in a different location (Lisbon, Porto etc.). Sometimes, a short ride on the train instead of a different drop off location, or going into town before picking the car up, could save quite a bit of money.  
•  TIP 3:  You never know what to expect of the driving culture in a foriegn country such as Portugal, but most drivers get the hang of it in no time. It could be very helpful to know where you are going (which seems obvious) so that you can plan you route in advance. Directions are mostly well signposted (except maybe sometimes in the big cities), but a map is still the best way to get around by car. 
• TIP 4: Always make sure you know exactly what the offer you have accepted includes! You can suddenly find yourself paying extra fees on a car you're picking up, which would make another all-inclusive option that you had declined more economical.