There is a Colt Ace Conversion kit and a Colt Ace complete gun. The original conversion kits (only 112 made) run from $1200~$5000, depending on sights, condition, Colt box, etc. The later model Ace Conversion kits go for $200~$500, also depending on condition , box, and Series.
The Pre-war Ace guns are quite scarce and can bring $1000 more than the same vintage 45 ACP gun. The later Post-War Ace Service model goes for about $200 more than a 45 ACP of the same vintage and condition. The frames for these guns were sold with the gun, not sold as a conversion unit.
Magazines are valuable also. The conversion kit comes with one mag so add about $40~$50 for each additional Colt magazine in excellent condition..
Give us a little more info please. Series? Commander? Full size? Box and accessories? Condition?