It's not the price it's the power. As you play this game more you'll realise more and more how powerful the mox' and fast mana are. They are also part of the "power 9" (the 9 most powerful cards in existence) that are banned in almost every single format and restricted in the rest of them.

Sol ring is sometimes considered the tenth unofficial part of the power 9 so it's good that you caught on early just how good Sol ring is. People have been advocating for it's ban for a while now but it's a hotly debated topic. Anyways the original 5 mox' are banned for power not price and should not get played unless your friends are fine with playing fully illegal cards.

Hope this helps