 Everyone loves a bargain and typically most will settle on the lowest quote. Unfortunately contractors offering incredibly cheap prices have to cut corners in order to turn a profit. There are numerous ways a pressure cleaner can cut corners. These may include using non-commercial pressure cleaning equipment, inferior detergents or simply not being insured specifically for "pressure cleaning"
When using non-commercial equipment a pressure cleaner can reduce his costs by having fewer overheads. Smaller machines require less capital to run (fuel), experience less wear and tear (replacing parts) and cost substantially less to maintain (periodical service which is generally every few months).
Universally accepted, a non-commercial machine is just not capable of cleaning to the degree a commercial unit can! Small machines just do not have the PSI or the much more important Flow! Beyond the machine itself, there are then the accessories. Accessories such as a surface cleaner (see video) are absolutely essential in preventing zebra marks.
Another way to cut corners is to use inferior detergents / chemicals or ...simply none at all! After all, detergents cost money. Make no mistake, pressure cleaning is a combination of detergent / chemical application, elbow grease and high pressure water. Eliminate any one of these factors and you can be assured of an average clean! | 
d is permit approved.