For a $100 budget, the playing field is HUGE. I have had great success with both Innokin products and the Joyetech eVic.
I would suggest you look into the eVic, at around $80-$100 depending on where you get it. My eVic has been my goto workhorse since I got it, and it has NEVER let me down.
If you want to keep costs lower then the iTaste VV v3 is awesome, but it is a smaller battery, only 800mah, although you can likely get 3-4 of them at the $100 budget to make up for the smaller battery life. I also really like my Sigelei Legend, it is SOLID in my hand and feels like it will take a beating. The downside to the Legend is the fact that you have to shake it to get into the menu system, if anything is going to fail I suspect it will be this "gravity" sensing menu system. You could also look at the Sigelei Zmax (but stay away from the Smok Zmax), either v3 or v5 would probably serve you well.
If you don't mind a box mod, then the iTaste MVP or the VTR are both exceptional as well.
As always ... YMMV!
HTH