Hi, just picked this up for $7.50. Wasn't really able to determine it's worth. Only a few pictures on image search, and nothing on ebay. They had another 2 at the thrift store, one with the same Lunar Lander sampler, and one with a different game sampler.
That is a nice find. There is a section on BallyAlley.com with pictures of the 300 baud tape interface:
You can load programs into this interface using the original Bally BASIC cartridge. This one:
The later released version of Bally BASIC (usually called AstroBASIC) has a built-in 2000-baud interface and isn't compatible with the Bally BASIC tape interface. This is the AstroBASIC cartridge:
Quite a few 300-baud programs have been archived on BallyAlley.com, here:
The tape interface was often hacked to allow it to be hooked up to a serial printer. Instructions for this are contained in the Bally BASIC Hacker's Guide by Jay Fenton, written in 1979.
Jay Fenton wrote Bally BASIC (and, later, the updated AstroBASIC and Hot Rod BASIC), as well as several arcade games, such as Datsun 280 Zzzap, Checkmate, Gorf and The Adventures of Robby Roto.