There are relatively few doorway chin-up bars on the market, but this article goes through the best ones. They are very reasonably priced, and very useful for arm and back strength, so read on to see them.
All-In-One Doorway Chin-Up Bar
The All-In-One Doorway Chin-Up Bar is very flexible, with many different grip positions for chin-ups, pull-ups, and narrow or wide-grip variations. As long as your door frame is not of a very low quality, this All-In-One Bar will do you well, and for only $30, it’s cheap too.
Personally, I used this bar until I upgraded to a power rack with steel bar attachment for pull-ups (which cost $1000 for the full rack, barbell and plates).
GoFit Chin-Up Bar
This bar is a little more solid, good for those people who can put lots of screws into their doorway. The limitation is that is only supports regular straight grips and your grip and elbows will be limited to the doorway width. So really, this bar is suitable for chin-ups but not suitable for wide-grip pull-ups (my elbows hit the door-frame when doing pull-ups). Due to that reason alone, the first bar is superior to this one, but if your intention is to do chin-ups, then this one is good. This one is $18.