Hi Jman,
There is a free software program called Digicamcontrol, that provides tethered control of various cameras, including many Nikon cameras. The program says it provides tethered control of the D3100, but unfortunately no live view or Bulb mode. If you had the D3200 or 3300 you would be OK in those respects.
Anyway, for what it's worth, you could try tethered control of your D3100 within certain shutter speed parameters (Probably only those on the camera) to get a feel for things, without spending a cent. The no live view via the software view is a downer though and no different as I understand to that provided (or should I say not provided) by Nikon's own capture software (Capture or Capture 2 I think they're called).
EDIT: You can get a D3200 or D3300 in the $200-300 range 2nd hand, if you look about. Much easier. See digicamcontrol supported cameras list:
http://digicamcontrol.com/cameras
Best
JA