Raspberry Pi doorcams
Yes, but you have to have a monitor to hook up, and they're $60 (or $80 for touchscreen).
...Unless you want to drag around a desktop monitor and hook it up temporarily (and they need A/C power, too) – but if you're going to go to that length, it seems like it'd be easier just to connect to the camera feed from a regular phone or tablet. There'd inevitably be some lag, but then again you won't be trying to carry a monitor around with you while you're adjusting the camera.
(Remember also that you may want to put cameras in places that aren't necessarily easy to get to, so having to hook something up while adjusting it – and then unhook without messing up your careful adjustments – could be a major PitB.)