My experience: I have health insurance, but it is a high-deductible plan, so I am effectively a cash customer as well. I went to UMC Quick Care a couple times, but when I told a coworker how much I'd paid there she freaked and told me that there was a much cheaper walk in clinic in town: Legacy Urgent Care.
UMC Urgent Care office visit cost: $204.
Legacy Urgent Care office visit for the exact same thing: $75.
Also, I was never able to get a straight answer in advance from UMC about how much a visit or tests would cost. Legacy, on the other hand, was able to give me dollar amounts or cost ranges over the phone. This indicates to me that they have a lot more experience working with cash clients than UMC does.
More Legacy price info: I recently took an intern (without US insurance, so he had to pay up front) there for an injured wrist, and it cost him less than $200 for the office visit, x-rays, and splint. That is relatively cheap! He was very happy with the care and price, so much that when we got back to work he went around recommending Legacy to people. (And he is from Germany where they have socialized medicine.)
Not only is Legacy Urgent Care cheaper, but I have found the waiting times there to be MUCH shorter than at UMC.
Legacy is at 105 N Pecos Rd, Henderson, NV 89074 (NW corner of Wigwam and Pecos). No appointments (walk in only) but you can call ahead to get their hours, directions, an estimate on how much your visit might cost (it will depend on what sort of tests they have to run etc.), etc. at (702) 263-4555.
On a related topic, if your job doesn't include health insurance as part of your compensation package, you can still buy it on your own. I wrote a recommendation for an awesome health insurance broker here recently: http://community.livejournal.com/lasveg