Canon have a speedlite for $100 but it has no swivel, bounce or zoom so it's very limited. Ideally, you want something that supports:
Variable Power - this allows you to change the strength of the flash output
Swivel - this allows you to swivel the flash horizontally so you can bounce it off the walls to your side or hopefully even behind you
Bounce - this allows you to point the flash vertically, either straight up or a choice of angels inbetween so you can bounce the flash off the ceiling
This one seems to have all 3 for $100:
http://www.bhphotovideo.com/c/product/542936-REG/Vivitar_DF400MZC_DF400MZ_Digital_TTL_Shoe.html#specifications
This one is slightly more than $100 and MAY have variable power, you'll have to look into it further:
http://www.bhphotovideo.com/c/product/605922-REG/Bower_SFD926C_SFD926C_Digital_Autofocus_TTL.html#specifications
Alternatively, you can check out used equipment but I didn't see any on B&H or Adorama that fit the bill. They mostly seem to be pre-digital units so they would not support E-TTL on a digital camera. They would function on manual but it's good to have the E-TTL option.
So it looks like the Vivitar I linked to is the most promising.
