Shooting is like motor sports and the old saying goes "speed costs money, how fast do you want to go?" How much do you want to spend is all relative to what your trying to achieve. I spent less than $1000 to get in to 3gun and used an old 870 I have, I didnt feel out gunned in the least and it took me a few events to learn what I liked/didnt like and I made informed decisions from there.
What do you already have? What division do you want to shoot in? Do you plan on traveling to any major events? All of these quesitons need to be answered but the bottom line is you need trigger time! All the bells and whistles arent going to mean much to you at this point, get out and shoot some events spend your money on ammo and practice, practice, practice!