Is corten steel expensive? Yes, it is. Corten is significantly more expensive than other common grades of steel. Corten steel planters are easily 50%-100% more expensive than a similarly sized planter of another long-lasting material. Custom applications will cost even more, since they need to fabricated and then installed.
It takes time to develop the patina. Some modular corten steel products (like fire pits) may already have the rust coating. Usually, however, they don’t. You will need to wait for it to happen naturally or accelerate it with an acid based product (or some diluted vinegar). During the process surrounding materials can be stained. If you have corten troughs, for example, sitting on a paver patio you will want to lift them by putting something under the edges of the trough so the pavers are not damaged.
Corten steel is long-lasting, but other materials last longer. Depending on the application and location corten steel can last for decades. However, in the Pacific NW you will probably get longer life out of stone and concrete. A juniper wall might even outlast a steel one!