United splashed out on Zaha in January before loaning him back to Crystal Palace for the remainder of the season.
Ince Snr told the BBC: "If you're talking £15m for Zaha, Thomas has got to be £25m. If you compare the two, Thomas should cost more. You're not going to get it because he's not proven in the Premier League but, if United are paying £15m for Zaha, on the stats, you'd have to say Thomas is worth a lot more than that. That's my opinion."