Streetcars are cheaper because of their lower infrastructure requirements. Often there is no need to relocat utilities, right of way does not need to be purchased and the stops are smaller and the vehicles more pedestrian oriented. Streetcar stops are also closely spaced if the goal is to be a circulator or short line transport mode. However if a longer distance transit mode that mimics light rail is what you're looking for, but your city is on a budget, the rapid streetcar might be your choice.
Many cities have taken up the mantle of the rapid bus to be their cost effective alternative to light rail, but only do this based on cost, not because its what the citizenry wants. Recent Rapid Bus movements in Oakland, San Francisco, and Charlotte have shown that people really want light rail on a budget but haven't been able to engineer their systems to reduce costs and are therefore left with an inferior transit mode for their stated goals.
But by using streetcars in center lanes with single tracking and passing sidings at stations you can get the same performance as light rail on 10 minute headways. Streetcars aren't single vehicles either. Skoda streetcars have couplers on them as well that would make them multiple car consists. The lighter vehicles are about 66 feet long as opposed to 90 foot LRVs yet you can still get increased passenger capacity and lower infrastructure needs. You can see in the picture below from Skoda.
This fascinating development in value engineering is nothing new and has been rarely used in the United States if at all. A recent extension of the Portland Streetcar to Lake Oswego might be its first test. Literature on the subject has been presented at TRB by Lyndon Henry and has been extensively covered by Light Rail Now! Recent publications including Raise the Hammer in Canada as well as the folks in Kansas City have been looking to this option. This technology and engineering arrangement is a smart way for cities to get rapid transit and build the system they want and can afford, not the system they settle for.