Real question is, how much will it cost to keep the Mrs. and children occupied?
Tons of malls in Atlanta, then there's the aquarium, the zoo, museums...
Also, are you driving or flying, Evergore? Myself and friends have made the pilgrimage by car from Baltimore the last 3 years and will be doing so again this October. If you are driving from Philly, think about how many times you will be stopping for food, breaks, etc. - usually we leave from Baltimore late on Wednesday night/Thursday morning, don't really hit to much traffic, make about 3 (maybe 4) stops along the way and arrive in Atlanta by about noon or 1 pm on Thursday. Account for lunch stops, gas and other spending along your journey if you travel by car.
As far as spending at the show goes:
Merch - Anything and everything metal related you could imagine will probably have a good chance of being there: CDs, DVDs, T-shirts, posters, etc. Plus most bands also bring their own merch. Be prepared to find a lot of hard-to-find or internet-only finds in the merch room. There is an ATM in the venue, but it's a good idea to pay cash so you don't have to pay service charges for getting to your own money. I make a list every year of bands I want to check out and CDs and DVDs that I want to buy. That might help you limit your spending a little.
Drinking - How much do you drink at a normal show? Would you like to have money to buy band members drinks if you meet them or get talking? (Trust me - it happens!)
Food - The venue does offer some basic bar food and their are a few other resturants within walking distance. Will you bring food with you to your hotel and cook there? Or will you be eating out for all your meals? Most hotels offer complementay breakfast of some kind, but lunch and dinner are up to you.
Beyond the cost of tickets and hotels and maybe food, the cost of your trip is pretty much up to how much you are willing to spend.
Me - I typically try to save up enough to take between $400 and $600 to ProgPower each year, but I certainly wouldn't have touble spending more than that if I actually had the money.