How much would it cost to protect the environment?
What will cost how much - environmental conservation or your project? The project cost will depend on the nature of projects you take up. It is possible to undertake very simple project like writing an essay on environmental protection, or a very elaborate one. A better way to settle the question of project cost will to first finalize the effort, time and money you are willing to put in, and then look for a project which fits the bill.
If you are asking about about cost of environmental protection, again there are no simple answers. There are no clear guidelines on what is environmental protection, and what is just a normal industrial or economic activity. For example, whether the cost of garbage collection and disposal system should be included in the cost of environment protection.
Also we need to look at the cost of not protecting the environment. It costs additional money to control emission of gases that cause summerhouse effect leading to global warming, but if we do not spend the money the cost of damage caused to the humanity will be many times more.
Just to give you a rough idea of money spent by various countries on specific programs for environmental protection will definitely be much more than a few billion dollars, but perhaps not as high as trillions of dollars.
There are so many things you can do with the environmental sciences that a cost is hard to say. Is this an experiment, research study, proposal? You could make a model to show ways to lessen erosion in coastal areas for the prices of sand and things you may have around the house. Study/observe animals in environments with high levels of pollution and low levels for the cost of transportation and maybe film. Is there more information about the requirements of the project that you could share?