We don't know the EXACT economics. Riot keeps that knowledge tightly under wraps. What we've surmised over the past nine years, however...
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"Popularity" is based on a number of factors - How many players, worldwide, own the champion in question? How excited are players to see a champion receive a new skin? Which champs have sold the most skins in the past? These matter the most to the Skins team and the company's evil and greedy Marketing department. Oh, and their Chinese overlords at TenCent Games! ;) That said...
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Popularity has very little to do with the current game meta - Decisions that are made separately from the skins department determine the game meta and this can change between patches. Urgot got a recent skin because of community pressure over the waits for Sion and Yorick. Heimerdinger was already on the schedule for 2018 and the timing is coincidental.
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How easy is the champ to fit into a new skin line? - Frankly, if a champ can easily fit within multiple genres, they're going to get a ton of skins. Ahri, Janna, Lux, Ezreal, Katarina, Miss Fortune, Twisted Fate, etc., these champs are going to get multiple skins because their models or characters can easily be made into multiple skin lines.
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How easy is the champ to work on? - Most of the above champs have easier models with few annoying particles. The team has learned to make skins for these champs relatively quickly and bug free (with some exceptions). Counter to that would be a champ like Yorick, who has multiple models in his kit (ghouls, wall, Maiden, mistball), and is considered "difficult" to make skins for. That, and the skins team has nothing but contempt for Yorick and his players.
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Where is the skin being marketed to? - Less than a year after receiving an Ultimate skin, Lux received the Lunar Festival version with Warwick and Nasus. Why? Because players in Eastern regions liked Lux more than a "more deserving" champ like Taliyah. Ahri, being a sexy female based on Asian cultures, has the same appeal around the world. Same with Miss Fortune, Katarina, Sivir, etc...
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Do they want to make a skin at all? - End of the day, the team is going to work on what they want to or what they're ordered to. If they're not ordered to work on Camille skins, it's not going to happen.
I know this wasn't reassuring, but it's how Riot has presented themselves over the years.
and
. But how do they define "Popular" if a champion ahem 
gets a much higher ban rate than their pick rate it means they're cancer as fuck and nobody wants to deal with them. Yet either A) they get a skin almost every year. Or B) Get a second skin surprisingly early after their initial release. 
got a second skin the same year of her release. Yet, 
who has been released about 2 years yet still remains a highly contested top laner AND jungler to this day. Yet she got no skin. 
has dominated in Winrate for this entire season. She hasn't got a new skin in a long time. Yet 
and 
dominated for only a few months yet got a new skin recently. I am honestly so confused on how things work behind Rito. Especially in their balance team.