Hopefully these chips will allow me to daisy chain my tlc5940s out to a usable distance.
OK that sounds alright.It is called just one way, not half duplex if the slaves never talk back.
The letters after a part number normally specify the package they are in and the temprature range they can operate at. Sometimes it will also denote the speed.Are they that cheap? They can be if they are surplus, out of spec devices from the reject bin, or have not been payed for. Bulk prices are often quite cheap and $0.20 each for that type of chip looks to me like it is the 100,000 up price.