Nice. This is the desktop version rather than the all-in-one portable? Whatever you do, don't seperate the keyboard and monitor, they are proprietary and useless by themselves.
I haven't seen one of the desktop versions on eBay in ages. Really no idea what one would go for today but off hand I'd think at very least a couple hundred. It really depends on how badly someone out there wants one. If you are in no hurry to move it you might start high (although people here might complain) and slowly reduce the price.
What condition is it in? Most I have seen are badly yellowed. A keyboard that is still white would be a plus. Also, is it the mono or color monitor? Obviously color is more desirable. Are there any other expansion options installed? Did you find any software with it?
Anyway, the TI Professional Computer is an 8088 based computer that ran an adaption of MS-DOS but is not at all IBM PC hardware compatible. That means most DOS software had to have special versions designed specifically for this machine, and it can not boot IBM PC DOS or vanilla MS-DOS. It's key feature was higher resolution video than the IBM PC. The system was available as both a desktop and as a portable, both with optional color or mono video.
As a hardware-incompatible, they did not survive long in an IBM PC market. They are rather uncommon, and today they would mainly appeal to fans of Texas Instruments or oddball computers.