Price sensitivity regression model

#1
I am trying to run a regression model that shows impact of price change to revenue. Unfortunately, price change only happens once a year and I only have 4 years of monthly data. What's the best way to find the impact of the price to revenue? I am faced with seasonality (December) too.
 
#2
I dont understand "impact of price change to revenue" i suppose its to predict the next price, and to only have 4 data or 4 x 12 data "four years of monthly data". Plus without any more info its hard to visualize what model would fit. Also revenue could affect price among others things.