Determining the most cost effective option to power your shop depends on a number of variables.
If the combined load of your house and shop is 200 amps or less, you could use your existing service to run a sub-panel in the shop. You indicate that you have two 30 amp breakers and one 40 amp breakers in your house. These "heavy" circuits would power large consumption items like a central air conditioner, electic water heater, electric stove, or electric clothes dryer. If all these appliances were running at the same time, they would could use one half of your existing service by themselves. However, it is possible that you have wiring for electric appliances but actually use gas for water heating, cooking, and clothes drying. This would free up a large amount of power.
You don't indicate the maximum power you might need at your shop. If this is a one man shop, you may rarely be running multiple tools at the same time and electric consumption might be minimal---especially if the tools are extremely light duty. However, I suspect that this may not be the case. It is likely that a person who has such a large shop will have heavy duty tools, will keep the entire shop well lighted, and will sometimes have multiple tools running at the same time (A multi-horsepower dust collector will be running at the same time as the table saw. An air conditioning unit, water heater and/or air compressor may also be cycling at the same time.)
If you need more electricity than a branch from your existing service can provide, the choices are either to run a second service to the shop or to replace your existing service with a 400 amp service located at either your house or shop and run a sub service to the other building. As others have pointed out, adding a second service (meter) will almost surely cause you to pay a second "minimum" meter charge, and may make the electricity more expensive (Some, but not all, utilities lower the unit price as consumption increases. If you add consumption to an existing meter, it will be at the lowest rate. If you add a second meter, consumption on this meter will start out at the higher rate.)
Another consideration is what you electic utility will charge you to either add a second service at the shop, or to upgrade your service at either your house or shop. If you run a sub-panel in one building off a service in the other building, you will have to pay the entire cost of running the wire between the buildings. If you have to upgrade the existing service in order to have enough electricity, the utility will have to run larger wires into the upgraded service panel. The utility may or may not charge you for this service. Such a charge could be substantial--especially if your house is a long way from the utility's power lines and if your electric service is underground. If you add a second service to your shop, the utility may or may not charge you for installing the new service.
I would suggest that you need to discuss you plans with your electric utility and determine their rate structure and charges for adding a second service or upgrading your existing service. You then need to discuss the options with a licensed electrician and get an estimate of his charges for the various options. You also need to consider the costs and/or convenience of locating wires for each of the options.
Thanks
How can I determan how large a service is on my house?