The cost of the valve is often small compared to the trim! I recently replaced my mother's tub/shower fixture. The rough-in valve was about $40, the trim was $160. This was a Delta (they also have a very wide range of prices and styles). On Delta, at least the version I bought, the cartridge comes with the trim. From a practical viewpoint, though, a new valve body means possibly a plumber, so yes, it does cost a lot to change out verses just changing the trim.
If you can identify the valve you actually have, Moen can help you figure out what will fit. A picture of the valve and trim could help those here. Depending on exactly what you have, you may or may not be able to just replace the trim - they have more than one valve, and the trims are generally not interchangeable between dissimilar models and definately not between brands.