There are a lot of ways to calculate the benefit of bike-sharing programs. For Paris' Velib bike-sharing program one could look at the number of Parisians using it, improvements to traffic congestion, improvements to the air quality, health benefits of the users, and so on. Adam Stein of TerraPass, a carbon offsetting company, crunched some numbers based on these stats from a NYC article on the Paris Velib program and came up with an estimate of the amount of greenhouse gases avoided. With that he concludes that Velib is an expensive way to offset carbon.

• Riders took 27.5 million trips in the first year.
• The current pace is about 120,000 trips per day.
• The program includes 20,600 bikes.
• The 1,450 self-service rental stations are available every 300 yards.
• The bikes are heavy and expensive — $3,460 and 50 lbs — built to withstand theft, mistreatment, and heavy riding.
• Nevertheless, 3,000 bikes have gone missing, about 15% of the total.

Velib cost around $142 million to set up with millions of dollars in operating costs per year. Stein, who happens to run a carbon offsetting business, does napkin calculations that the Velib program reduced around 40,000 metric tons, "about the amount saved by removing 5,700 cars from the road". A price of a tonne of carbon, in his estimation, comes to "hundreds of dollars, a price that makes solar photovoltaics look like an incredible bargain".

That's a big number! Especially when the going market price of a tonne of carbon is somewhere around $30. Stein, however, seems to assume that the money invested is just being used to offset carbon emissions. (Not to mention Velib's side benefits of reducing traffic congestion, improving air quality, improving human interaction) Velib is a transportation service first and foremost that people pay users fees to use. JCDecaux is not a charity nor are they investing money purely to reduce greenhouse gases. They are providing a service and making a profit through user fees and advertising revenue.

I think an argument could be made, however, that the advertising revenue is actually a government subsidy since the City of Paris is giving up the rights to that revenue stream in exchange for the bikesharing project. So that portion of the ad revenue that is helping to pay for Velib could be considered a payment for the public good, including reducing GHG, improving air quality, reducing sprawl. But I'm pretty certain that JCDecaux found a way to make the Velib program to mostly pay for itself even if they lose over 3000 bikes per year.

I think you could say that Velib is a very affordable way to reduce GHG because most of the carbon reductions have been a happy but incidental benefit to the main purpose of providing alternative transportation.