Using present technology, estimates of sequestration costs are in the range of $100 to $300/ton of carbon emissions avoided. The goal of the program is to reduce the cost of carbon sequestration to $10 or less per net ton of carbon emissions avoided by 2015.
Assuming $10-100/ton: Sequestering 1 billion tons of carbon dioxide would cost $10 to 100 billion per year.
Methane represents 18% of the total annual greenhouse gas effects Methane has 25 times the global warming effect as an equivalent amount of carbon dioxide
Sequestering 40 million tons of Methane would be the equivalent of sequesterint 1 billion tons fo carbon dioxide.
The sources of methane and the natural sinks for methane are described at this link.
From wikipedia sources and sinks for methane emissions
We are adding +20 million tons per year of methane.
Slightly over half of the total emission is due to human activity.
Living plants (e.g. forests) have recently been identified as a potentially important source of methane. A 2006 paper calculated emissions of 62–236 Tg a-1, and "this newly identified source may have important implications". However the authors stress "our findings are preliminary with regard to the methane emission strength". These findings have been called into question in a 2007 paper which found "there is no evidence for substantial aerobic methane emission by terrestrial plants, maximally 0.3% of the previously published values".
Long term atmospheric measurements of methane by NOAA show that the build up of methane has slowed dramatically over the last decade, after nearly tripling since pre-industrial times. It is thought that this reduction is due to reduced industrial emissions and drought in wetland areas.
Recent experiments suggest that forming methane hydrates is fairly easy
Here is more information on methane hydrate
Open the Future discusses that each cow produces 110 kg of methane in manure each year
Oil rigs can pump oil up from under a mile of ocean. If we used a reverse of this method a pipeline and ocean oil can be used to send Methane to the bottom of the ocean. Deep ocean oil platforms cost about $100-500 million Oil platforms can move 50,000 barrels of oil per day and 2 million cubic meters of gas. About ten barrels make up one ton. About 2.3 million tons of methane could be sent down one oil platform each year. Twenty oil platforms would be needed to sequester 40 million tons of methane. This would cost between 2 to 10 billion dollars. This is less than the low cost target for carbon dioxide capture.
Methane can be more easily captured in the form of livestock manure and bird poop. Some of the useful fertilizer can get extracted, but we could take methane and convert it into methane hydrate by pumping it onto the ocean floor. Methane hydrate is stable on the ocean floor now. 40 to 200 million tons of methane could be diverted from the generation sources we have now. We could go into a net negative for methane of 20 to 180 million tons. It would be a simple way to start curbing greenhouse gas effects and offset global warming and climate change.