300m gets you:
MP10 barb
MP8 monk
MP8 wd? maybe
~MP7 wiz or if you do low DPS CM you can play support up to MP10, maybe
MP5 DH
Probably not full farm spec but you'd be able to survive and depending on the class, contribute to a team
A lot of where you'll fall between MP7-10 will depend on how lucky you get when you buy your weapon and how lucky you get on your main craft spots (bracers, amulet, gloves, shoulders)
Here are my alts for reference:
http://d3up.com/b/4880/barb
http://d3up.com/b/481048/bast (dh)
http://d3up.com/b/61233/wd
http://d3up.com/b/484553/logen
I'm pretty sure I geared most of them for under 100m, probably around 50m/ea although the difference between 50 to 300m isn't that big outside of wearing 50 mil in gear and having 1-2 120mil weapons. I can do MP10 on DH, WD (firebats), MP4ish archon sharing some WD gear or MP5-7 with CM gear (in profile), barb is mp10 of course
edit: just realized my DH has 160mil in gems. Just ignore that; the gear was cheap! lol