When you compare fares between several different airlines one new fee you'll need to factor in is the cost of checking your baggage. It used to be free to check 2 bags with just about every airline. Now your luggage, if you check 2 bags each way, could add $110 to your round trip cost.
Below is a ranking of major US airlines based on their baggage fees. These are current as of January 13, 2010. Keep in mind these fees are EACH WAY, so if you'll have the same luggage going round trip then double the fees below.
1. Southwest: First bag is $0, Second bag is $0. Two Bags: $0.
2. JetBlue: First bag is $0, Second bag is $30. Two Bags: $30
tie 4. Alaska: First bag is $15, Second bag is $25. Two Bags: $40
tie 4. AirTran: First bag is $15, Second bag is $25. Two Bags: $40
5. United (prepaid online): First bag is $15, Second bag is $25. Two Bags: $40
6. Virgin America: First bag is $20, Second bag is $20. Two Bags: $40
tie 9. United (paid at the airport): First bag is $20, Second bag is $30. Two Bags: $50
tie 9. Frontier: First bag is $20, Second bag is $30. Two Bags: $50
tie 9. American: First bag is $20, Second bag is $30. Two Bags: $50
tie 13. Continental: First bag is $23, Second bag is $32. Two Bags: $55
tie 13. Delta: First bag is $23, Second bag is $32. Two Bags: $55
tie 13. Northwest: First bag is $23, Second bag is $32. Two Bags: $55
tie 13. US Airways (prepaid online): First bag is $23, Second bag is $32. Two Bags: $55
14. US Airways (paid at the airport): First bag is $25, Second bag is $35. Two Bags: $60
15. Sprit Airline: Spirit gets the lowest rating because they charge for both carry-on and checked bags. Their fees if paid in advance are $30 for a carry-on and $25 a checked bag, and if you pay at the gate the fee is $45 each.
With some airlines there are other ways around the checked bag fees. Some of the airline credit cards, which often carry an annual fee, have the added perk of letting you check 1 bag for free.