The most expensive operation is the dismantling and manual cleaning of the valves. This is what most mechanics will recommend. The price will vary according to the brand and engine type, however it is the cost of labor that takes the biggest financial toll since the valves are so difficult to access.
The least expensive option, with no need for disassembly and entailing the shortest service time, and which is an alternative to part replacement, is hydrogen treatment via the Carbon Cleaning station. This service works as a preventative treatment, recommended every 15,000 miles, but can also resolve existing problems in the engine.