Actually it wouldn't cost that much more to extract DNA from urine than it would from anywhere else in the body, including from saliva. With the proper reagents, DNA extraction is pretty routine.
If you're talking about SEPARATING salivary from urinary DNA, there's no extraction method that will do that--every available method is based on the physicochemical properties of DNA, which are identical between individuals and identical throughout all organisms--IOW, genomic DNA is genomic DNA, regardless of where it came from, and there's no extraction method that will distinguish between DNA from different sources. If you think say, the salivary DNA came from a different individual than the urinary DNA, then distinguishing between them would have to come AFTER extraction and purification. And that would be best done using specific primers for individual DNA markers using PCR--that's how forensic DNA analysis is done.