My rates for Re-Stretch are $125/room or area.
While my rates are similar to others, I do not take the shortcuts that others do. I use a power stretcher in every part of every room. Others use a knee kicker to smooth out wrinkles to make it look flat without being tight. With power stretching, it is done once, and not needed again for the lifetime of the carpet. I am one of the few who will work with a room full of furniture, moving the pieces right and left, then back into place without an extra charge...and I have the specialized skills and equipment to reverse the stretch to take the excess out of a doorway seam should there be "permanent" furniture that cannot be reasonably moved.
Keep in mind that when the wrinkles are stretched out, sometimes other issues may become visible. Stretching carpet does not magically fix poorly executed seams by original installers, irregularities in wood and concrete subfloors, feeling cables laying under the carpet or distressed pad that may be visibly thinner in walkways and give the illusion of a “roll” in the carpet.
Loose carpet is hard for a professional to clean pulling and pushing the wrinkles… Pushing and pulling the wrinkles also breaks down the backing, reducing the life of the carpet by years. It is also hard to remove water from a loose carpet, resulting in it being wet for two or three days instead of 4 to 8 hours after cleaning . When carpet stays wet, the moisture goes down into the backing which is often made with a lot of latex and clay, which can be broken down further by water also.
Short answer: While I like working on clean carpet it is better for the carpet to be stretched before it is cleaned.