I don't have any Monacos, but can offer some personal observations on the desirability of the movements, particularly as many of them are shared across the Heuer range.

Where the same model was offered with manual and automatic movements, the automatic typically seems to command the higher price. My collection spans both and I don't really have a distinct personal preference but there is something iconic about the micro-rotor autos in particular - the fact you can see at a glance what movement is inside by the left-hand crown helps in that regard.

When it does matter is where there is an iconic model within the range. Take the "Jo Siffert" Autavia, for instance. I've seen manual white Autavias with blue accents for sale described as Sifferts, but for the true collector it would have to be an automatic as that's what Smokin' Jo wore himself. Whether the true collectors are always the ones with the money is another question! The same would go for a McQueen Monaco - the current trend on eBay seems to be to describe just about any 70s Monaco as a "McQueen" but authenticity has a value.

As far as Cal 11 versus Cal 12, I don't think we typically see any consistent differential in price. What is important to me and (I imagine) for many collectors is that the movement should be as original as possible - in the absence of a handy online reference to compare movement reference to case number to check for originality, what becomes important in the Heuer world is the dating of the watch and thus what movements were possible. I can't check the specific movement against case for a Carrera 110.253, for example, but I know that the watch is late enough that it can't have had a Calibre 11 when manufactured. So a Cal 12 would be fine, but if it had a Cal 11, I would look for that modification to be reflected in the price. The Cal 12 is the "new, improved" version of the Cal 11, of course, but after the best part of 40 years, that is an irrelevance in terms of value and originality remains king.

At the time it was introduced, the Cal 15 was an economy version of the automatic movement, priced at around a 10% discount over the Cal 12. Availability drives the current prices of the Cal 15s - some of them are notably rarer than Cal 11/12s as they were in production for a shorter time and there is some value in scarcity. In part, the price is also affected by aesthetic considerations and those are entirely individual - personally, I like the symmetry of a Cal 11/12 dial but I have Cal 15 watches and enjoy them too. Typically, there aren't as many Cal 15 models - a vertical collector like myself might find it easier ticking off a couple of Cal 15 boxes than covering the variety of versions produced with the Cal 11 and 12 movements.

My main advice would be to study examples, read all the literature you can and get as familiar as possible with what it is that makes a particular watch original. And once you have one (or two, or three, or a hundred), enjoy them for what they are as well as just the investment aspect - the market is there, it seems robust but who knows what can happen nowadays.

Jeff has a price guide on the OTD site, but that's an incredibly difficult thing to maintain. When it comes down to it, any watch is worth whatever someone is willing to pay. Get two such people in opposition to each other and the price can sometimes skyrocket; find one that hangs around a while without selling and you may snatch a bargain (or a pup). I have an inner compass for what I think a given watch is worth to me but for the right watch I'm prepared to go that bit further and a big determining factor there is how likely I am to see another for sale and how soon that might be.

Now, over to the Monaco guys for their insights!

Mark