Real opal... You mean natural? Precious opal (gemstone quality) is very rare... far rarer than diamonds. They are also heat sensitive, have a propensity to craze, and are soft (5). Merely cleaning it will scratch it's surface and they shouldn't be worn very often. I don't think you'll find a natural opal ring for that price as just the stones sell anywhere from hundreds to thousands depending on color, clarity, cut, and size.
Solid stone synthetic vs terrible fake? I would stay away from the blue ones. Stay away from things that have black backings or sandwiched between crystal/glass (triplets or doublets). But it is largely going to be what you want. I have this and this synthetic opal rings. While both are solid stones, the quality of the second is better, but the former is more durable for day-to-day wear (has a plastic top coating to protect scratching). I have synthetics from higher end places as well, and I see no difference between them and the second one I posted. I have found that Amazon collection, in general, is a pretty decent place to buy them.
Other good places for synthetics will be at named stores (Kays, BlueNile, Jared, etc) or high end (designers, local jewelers, really nice sellers on etsy)