I don't see streetcars being much less expensive than LR, especially compared to LR on private R/W, except for the actual vehicles. Construction in streets is a true pain and very expensive in older areas where unknown underground utilities abound. So you really need to know the market and be pretty sure you need the level of investment that a streetcar requires.

For downtown/urban areas, I kind of like trolley buses for local service. They have some of the route flexibility of diesel/CNG buses, ability to climb steep hills, and zero emissions. SF has actually added trolley bus lines over the years, converting several diesel lines to electric. Streetcars have to work in traffic like the buses, so there's not much advantage in terms of schedule reliability. Of course, SF's trolleys (both tracked and trackless) have the advantage of low-cost electricity from city sources - electric anything would be dicey elsewhere in PG&E territory.