You might need a set of plugs, which are not expensive. Change the oil once or twice, keep the chain lubed and the air filters clean, and thats about it.
Figure in the cost of fuel and premix too.
If the bike is tuned right, and the cylinders/pistons/rings are in good condition, yearly maintenance will be very low. Banshee's dont REQUIRE alot of work, but if they are setup wrong, they can be a pain.
To put a $$ value........i'm not really sure. Compared to the cost of the bike and the mods, it is virtually nothing.