No, 737-400 compares with 737-800 (same size) 737-300 compares with 737-700 (same size) and 737-500 with 737-600 (same size).
I'm talking about the price difference.
B737-400 49.99m - 62.65m USD
B737-800 67.28m - 75.68m USD
B737-900ER 78.04m - 87.51m USD
So you compare -400 with -800, ok. Next time please be more detailed in such things
Fuel is insignificant compared to the 20 million dollar difference, maintenance is virtually the same.
No, 20 Million Dollar are (in my opinion) not this much. If you order an aircraft like the B737 you will use them till the end of the game, 20 years. But ok, I'm sure for some airlines 20 Million Dollar are much money, but than they don't have to decide which B737 they should by. In fact I'd use MD-80 or MD-90 over B737classic in this scenario and US domestic market.
That doesn't make a difference to me in the America domestic market.
You can use your B737NG to fly to Europe or South America and the bigger range is a pro argument. Next time please be more specific where are you based etc. if this is important for the question.
No. The B737-900 and B737-900ER have no equivalent in the B737classic series, so (for me) it's a very big point on the positive side.