And that's exactly why we haven't seen any alt-leader DLC packs. The amount of cost on the production end is the same, but the amount of value on the consumer end seems like it's less, even though it actually isn't.
One trick Firaxis could maybe use to get around this problem is to just include additional "bonus" content to justify the price in the mind of the consumer. Charge the same amount for alt-leaders that they would for full civs, but also include more scenarios, world wonders, natural wonders, city states, etc. than they did in the full civ packs. Basically, the same trick they already did with the Vikings DLC, which may have been a test case to see how readily consumers would accept that kind of content as valuable.