That would depend on few things. Take a full-on rainbow Morgan like you mentioned. They tend to bring in the neighborhood of 20% higher than an average mint state examples I've seen sold at Heritage, Great Collections and a few other auction houses. Second, if both sides are similar in tone then the price will increase. But mostly I think it's that certain segment of collectors who lean towards them. This applies to more modern coins as well. Especially Standing Liberty and Franklins halves.
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