Depends on the quality you're looking for... Making up some random task time-frames, we could say:
~50 Weapons, split between swords, staves, axes, etc.
~20 Different Enemies
~7 Unique Boss Characters
4 or 5 different "Platform" tile-sets (Different themes, one is dark ruins, one is grassy, etc.)
GUI elements for health, mana, actionbars, etc.
.. 1 day per weapon * 50 weapons
+ 1 week per character * 27 characters
+ 1 month per tile-set * 5 sets
+ 1 week of GUI work
= 290 days = 2320 hours of work.
@ $20/h that's about $46k over a bit more than a year. You could easily double or halve that number though, depending on the quality you need.