The valuation is actually quite high and in the range of a lot of web companies. Zipcar is not new and has established revenues and profits. With this buyout the company has a 112 P/E, comparable to what Netflix is valued at (116 P/E).

The difference with purely web companies is Zipcar has a lot of fixed costs, if they want to expand to double the amount of users they will have a ton of capex to double the amount of vehicles (yes, you can probably squeeze a few more hours of usage into a car, but like planes there is a fixed number of hours any given vehicle can be generating you revenue per month). Web companies also see infrastructure costs dropping over time while Zipcar sees the opposite (after accounting for economies of scale, cars are not getting cheaper year after year).