1 Unit(dont know if its actually short for Unit though🙂).
When talking about heigh in mount racks, you allways use U's to determine how high a server or switch or whatever is, 1 U is 1.75 inches or just under 4.5 cm's.
As for the original question, why do you need a cheap 1U solution?
Most people looking for rackmountable stuff in general are people looking for servers, and people looking rackmountable servers tend to have at least some money to burn 🙂