While not a member of the Power 9 family, Sol Ring is without comparison the most powerful card you can find for less than a Euro. And in some of the formats where you may encounter it, it is often judged as the strongest card available.
Among the widely played formats, only Commander and Vintage allow you to put a Sol Ring in your deck. Since Commander is a singleton format and because the card is on the Restricted List for Vintage, there’s no mainstream format where you can play more than a single copy of the card. This makes sense because if you've ever played a (or against) a deck with four Sol Rings, you will know just how explosive starts can get that multiple copies on the battlefield can create. Some seasoned players even unequivocally recommend picking Sol Ring above any of the Power 9, including the great Black Lotus, in Limited environments such as Cubes.
In Commander, Sol Ring is considered almost symbolic and hardly any Commander deck ever misses out on the opportunity to include this powerful artifact. Often when evaluating the course of a game of Commander, the outcome can boil down to someone casting the one-mana artifact on their first turn.
As expected from a card this popular in formats where players often put a lot of pride into blinging out their decks, special versions of Sol Ring command a healthy price. The card has been printed in four different illustrations and in more than 20 different versions, half of which were for Commander-specific products, featuring Mike Bierek’s near iconic depiction of the ring. Even more famous is certainly Mark Tedin’s "ring of fire" artwork that goes all the way back to Limited Edition Alpha (August 1993) costing you at the very least 2.000,00 € a piece. Unsurprisingly, the Alpha print is only surpassed in price by the exceedingly rare Summer Magic (July 1994) misprint of the card at around 4.500,00 €. The Sol Ring of Limited Edition Beta (October 1993) and later special versions such as the ones from Judges Rewards Promos (June 1999) and Kaladesh Inventions (September 1016) while still costly, seem quite affordable by comparison, costing you hundreds instead of thousands of Euros.
If you need a Sol Ring for a casual Commander deck or kitchen table play, your best bet is to buy one of the latest Commander or Commander Anthology printings. You will definitely end up paying a lot less than for an Uncommon card. But if you want to invest in a card that will make opponents, spectators, and friends drafting your Cube take notice, any special version will do.
Whatever you put out, you are very likely to get it back if you choose to sell your copy. The numerous Commander players around the world are almost certain to keep demand high and prices increasing, or at the very least stable when it comes to any of the less run-of-the-mill Sol Rings.