Cost of doing of allergen immunotherapy per year?

Varied. Really depends on your insurance coverage. Studies have shown that is definitely a cost savings over five years when taken into account medications not needed and secondary effects such as decreased infections and asthma exacerbations.
Immunotherapy costs. It is difficult to give an exact answer, since the costs of immunotherapy can vary depending on the number and type of allergens the person is being treated with, the frequency of the shots (initial therapy vs. Maintenance) and other factors, such as office and personnel costs which can vary by location. A rough range would be $30-50 a visit for someone getting two injections, including extracts.