They usually go for $100 - $130 on eBay for just the computer. The harnesses I've seen from $60 to $100... A local junkyard..er.. used Mustang parts supplier wants $150 for a used A9L. Dealer list for a "new" one (refurbished by Ford) is $479 so figure around $300 retail...
I wouldn't take $100 for both. You'll probably make $200 if you throw them on eBay.
With 200,000 mustangs if not more from 89-93 with mass air I really don't think any of the computers ford used are considered to be of great value. But that EEC will fetch you some good coin if you choose to play it up on ebay.