Tattoos are charged by the hour, so just going by the size isn't going to help. The complexity of the design will determine how long it takes, and therefore the cost. The tattooist will look at your design, guestimate how long that will take him to do, and charge you accordingly. $100/hr is about average.
Pick your tattoo shop carefully. Don't pick a design off their wall. The best way is to take something in that's the general idea of what you want, and have the tatooist draw up something custom. He'll make sure the design is a good one for tattoing. Things will too much small-scale detail will eventually bleed together and not look very good.
Here are a couple of my patriotic tattoos
[img]http://www.digitalpose.com/mbr/1/18107/p/328915_1281014639370617422_v.jpg[/img]
[img]http://www.digitalpose.com/mbr/1/18107/p/328916_2153703078869396789_v.jpg[/img]
this one's a shitty photograph, but the tattoo is of a B-24 flying through the clouds
[img]http://www.digitalpose.com/mbr/1/18107/p/387324_3844592691958150775_v.jpg[/img]