Suggest that you review the Colt Ace listings and sales history on gunbroker.com to get an idea of what folks are listing them for, and others are actually paying, the latter being the most significant. There's typically a big difference. As you're probably aware there's a wide variety of Ace configurations dating back to the 1930s and continuing with new manufacture into the 1980s. Those listing for several thou rarely if ever get a bid, while those at the lower end, typically get sold for semi-reasonable amounts starting around $1K.
The pre-war variants list typically for several thousand $$, while the latest versions list for a fraction of that--I assume because they're much less desireable as functional pistols and collector's items. I paid exactly $1000 for my Service Ace at a local gun shop. I hadn't done any history on them and it was purely an impusle buy, but believe I paid just about fair market value for it considering its vintage and condition.
Whether you choose to go the on-line auction route is your choice, obviously. My experience as a buyer has been all postive. A dealer is going to give you a fraction of its retail value. In my opinion the market for Colt Aces and conversions is pretty limited and only those true Coltoholics are going to pay the premium bucks.