Although Northrend was the last expansion and only 5 levels below us it is possible to solo more than a couple bosses however the real money comes from a quick and easy run by 2 - 4 people depending on the instance you will make a jackpot.
Every boss in Naxxramas yields 85g for a 3man team and every boss in there can be 3 manned by a number of different group setups. So do the math 85g x 15 bosses = 1275g just off of the bosses! 3 manning it myself changing between ret and holy with a BM hunter and another ret pally took just over 2 hours. On top of the gold looted from every boss you have the epic drops which if only 3 of you are there you can expect to get another 200g from, you have the trash drops the gold the cloth the greys the greens in 2 hours you can make 2000g a piece! This is a fun non daily or grindy way to make some good cash, some fights can be hard for a 3man or even a 4man team but they are all easy enough for 3 good players. If you chose to go with 4 people instead you will receive only 1500g but it will only take you an hour and 15minutes, this is more gold per hour but less gold overall. If you have any questions about how to take down a boss with only 3 people or perhaps how to solo a boss in Naxxramas just leave a comment and I will respond as soon as I can.