I don't think you will find a cheap price with that one way journey, relation for the company is going to be expensive and the is probably the most common journey for tourists to take.
As far as recommendations for a company, we have used: EZI, Jucy, RAD, EZU and Hansen (in Cromwell) in recent years.
Basically every trip we do we end up with a different company as no one seems to always be the cheapest.
Jucy used to always be pretty cheap but don't work if you need to do after hours pickup/return.
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