Vectrex is a great console if you like old vector arcade games. Definitely test first if possible, but the Vectrex is pretty reliable. The humming noise is normal. There are hardware mods to reduce the hum if you wanted to.
Systems are usually worth $100+ in decent condition. Loose games are usually $10-$15 and CIB games are $20-$40. There are only a few rare/expensive games, like Pole Position or Polar Rescue. The 3D glasses add-on and 3D games are $$$ and the light pen and light pen games are a little pricy too but they're both just collector's novelties and not much fun.
All of the original releases were issued as public domain long ago, so the only reason to buy original carts is for collecting and to get the tinted screen overlays. Otherwise there are several multicarts available that give you all of the original release games selectable from a menu.
Vectrex also has a lot of homebrew releases. Since all the hardware/developer docs were made public domain, Vectrex was one of the first retro consoles to attract homebrew development. Modern technology allows larger cart sizes so a lot of the homebrews are better than the original retail release games. Most homebrews are on-going availablility and are available cheap. A few had limited runs and are $$$. Check out vectrexcarts.com for some cheap homebrews. Gravitrex Plus, Protector/YASI, and Thrust are excellent. Some of the homebrews are available on multicarts too. (not those 3)