You may have seen a data sheet circulated recently for a Mouse 
interface IC from NEC, the uPD4701c.
This chip may be used for a single Trakball / dual steering wheel 
interface in place of the LETA chip. It accepts the same type of 
Quadrature signals that our optical coupler produces and counts all 
four phases of the quadrature signal (same as Leta Hi resolution 
mode).

Main Advantages of the uPD4701 :
No CPU wait states required.
No clock required.
12 bit accuracy.

Main disadvantages :
Only one Trakball (= 2 steering signals) per IC.

Costs :
$3.50 @ 5K, $2.75 @ 10K, $2.25 @ 20K

Last known cost on LETA was $2.60 @ qty unknown.

For more Information on the uPD4701C contact Erwin in componemts.