So basically 1/3 of the fleet is A319. My previous employer had a couple in 144 configuration which was tight enough. Ended up getting rid of them as the operating cost wasn't that far enough below the A320 to justify losing 36 seats, and it couldn't cover a swap when a full A320 went AOG.
Customised aircraft tend to have lower value on the second hand market as potential buyers are put off adding something which requires different training or spares holding compared to the rest of their fleet. Bog standard is much easier to sell.