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You haven't mentioned either of two 'hidden' costs of renting a car, and both of those are major. The first will apply whether you rent the car in Denver or Portland and that is the 'Extra Driver' charge. Since you will be with a friend and not a spouse, parent, child or (provable) business associate, this charge will apply if you both want to drive, even if one of you only drives for a few minutes. This is a per/day charge applicable to the entire length of the rental contract and thus might be more costly from Denver if you are adding days for the drive rather than subtracting them from your time in the Northwest.

The second charge, and it's substantial, but would only apply if you rented the car in Denver and left it on the west coast, is the one-way drop off charge. You need to check with the rental companies (the information should be available on their websites as you set up the reservation) before you make your final decision.

AZBuck