The battles in the Metro and Metro Station do make loads of money, same like those in a film studio! I really enjoyed No More Heroes such a fun game, with constant slaughter of countless enemies.

Make sure you finish all the non-assassination missions so that you unlock all the assassination missions. Do one of each mission and remember which one gave you the most cash. I finished the game with all personal upgrades and a few extra clothes. Only a few times it got annoying (then I bought the clothes). I think you should do the weapon upgrade. The one with three plades rocks!! :)

Just make sure you do each job well at least once to unlock more jobs. I forget which ones were early, but I remember the baseball ones not being unlocked toward the end, but some of the killing jobs that paid well were unlocked pretty quick. I should do a run through again the entire game was fairly short...

Just wish you didn't have to enter all these side jobs and side missions to make money and could just get on with the business of playing the main missions.

Don't buy every upgrade. Save some money and skip several. Early on they are not all unlocked. so don't think you are getting the best. I only get the upgrades when I am not doing well enough against the enemies or have tons of extra cash.