Rarity – As mentioned previously, the rarer a variant is, the more likely it is to be valuable. However, that’s not always the case. In ten years if someone looks at a 1 in 100 variant and thinks it’s poor artwork, they’re not paying you for that issue just because there aren’t many in existence.
Artist – Some artists attract greater demand than others, whether they’re great artists (J. Scott Campbell), or retired (Neal Adams), or they’re more known for other types of comic work (David Petersen). But, even with great artists, a poor cover or a common variant has lesser value.
Character – Some characters will usually always draw a larger attraction, such as having Venom on the cover (even when he’s not in the issue), simply because fans like the character. An oversaturation of characters can also have an adverse effect (such as Deadpool post-movie release).
Features – Once in a while, a publisher will release a comic with special features, such as a lenticular cover, gold lettering, hologram cards, that increases the value of the variant. The value of these is typically based on the rarity. For instance, the recent DC Flash/Batman crossover miniseries featured lenticular variant covers, but most comic book stores purchased as many or more than the regular cover because they knew they’d move. Those lenticular covers are primarily selling for cover price today, and likely won’t increase much, if at all in value. Meanwhile, Image Comics has been releasing one-per-store gold and silver lettering issues, which have sold for as much as $150 on the secondary market.
Quality – The most important feature of the value of the variant is its quality. Is it something you would want to display on your wall? Did they pay close attention to detail? Is it a virgin variant or sketch variant? Viewing the stock photo online is difficult to gauge the quality, so while you may want to wait until you see the final company before committing to purchase it.
Theme – Many variants are released based on certain themes of that month/period. DC did this often during the New 52 era, and I can’t think of many that are worth more than cover price. Marvel often has monthly and ongoing themes, and many times overdoes it. For instance, the hip-hop variants were a cool novelty at first, and then they continued with it until people lost interest and the value of all hip-hop variants has dropped. Meanwhile, they also do monthly themes, which are hit or miss. A few months ago, they did Venomized variants, which flew off the shelves, but then more recently they did Mary Jane Watson variants, which really came out of nowhere, and did very poorly in stores. Image tends to think a bit bigger with their variants. While also often doing monthly themes, their variants often have more thought put into them than tossing a Looney Tunes character into it, such as Images of Tomorrow (thought provoking), gay pride variants (socially focused), and 25th anniversary variants (nostalgic).
Age – A lesser way to gauge the ROI on a variant is how long it has been in circulation. Often, a variant will begin to lose its value by the Monday after its release. If by Monday it’s still going strong, it likely has staying power.