Friend of mine did that. Paid nearly 200€ a month (advertising included pictures of naked pretty girls. Yes, a night club.)
There are some agencies specialising in that kind of advertising, however, you have to be raher careful. There have been some cases of fraud, and of course you have to read the fine print very carefully. Often these agencies requires your vehicle to be rather new.
One of my comrades drove gigantic stickers advertising for the car dealer he bought the vehicle from. Turns out this almost offset his car payments for the first two years.