what are the costs of converting from gasoline to natural gas?

This is not a math problem, it is an economics problem. For an individual vehicle like a car, the equipment on the engine is a few hundred dollars, the same basic equipment is used in LPG powered vehicles. The tank is more complicated due to its size and what pressure the gas is compressed at. But it may be a thousand or so dollars. The big issue is providing refueling facilities, transportation and compression of natural gas in quantities to replace gasoline, and developing enough natural gas to replace gasoline as motor fuel. Those are social and economic issues that can be very expensive.

No one knows the costs because there is absolutely no reason to make the conversion. It is wasteful and not economic.

Now if you want to convert natural gas to gasoline that is another matter. Those costs are currently being developed but the companies doing it are not about to let that information out.

After rereading your question I am not sure if you are asking about converting gasoline to methane or converting a gasoline engine to burn methane. If the question is related to converting an engine then I have to agree with the first answer by charles s.

"Oil Field Trash" was correct..and there's no way you can know how much it cost just like that..

We have to determine the research that have to be carry out to make sure the conversion is liable for end user, the cost of overall process involving equipments that will be use, trained personnels, logistic ***/and custom matters and much more..

Gasoline to gas does not make sense in context of long term investment, but Gas to liquid or also known as GTL is a breakthrough in Oil & Gas industry. The first GTL product were produce in SMDS Bintulu, Malaysia and have been use in many countries. As far as I know Qatar has already start producing their own GTL..