As said, it depends on your location.
If you're on, or right off of a main drag, then it's probably not going to be incredibly beneficial for you.
We're inside a (very large) county recreational park, not only that but we also don't have a physical street address, so the best we can use for GPS directions is a cross street, 1/4 mile from our attraction. We're in hilly Pittsburgh, so even if you get to that cross street, you still can't see the haunt. We bought our spotlight a few years ago, I think this will be our 4th season for it. It has most certainly helped us, IMO. Getting IN the park is pretty easy, getting to us once you're in it, a bit more difficult.
Our spotlight was delivered to us in May of 07. Of course I didn't wait to fire it up, I wanted to see what kind of distances you could see it from. Literally minutes after firing it up, random people were pulling into our parking lot. As I said above, we're not exactly off of a main drag and we don't get a bunch of random drive by traffic, so seeing ~30 cars show up within the 3 hours that I had it on was pretty impressive.
The initial cost for a decent light is generally going to be right around the cost of a decent prop. I want to say our spotlight was ~$3500. It's a 3000w xenon arc with a 16" reflector unit with a motorized head. I would suggest 3kw as a minimum, hindsight being 20/20 I would have gone with a 4kw-7kw unit. My unit actually draws 3720w from the outlet after the power loss on the power supplies and the AC motor to turn the head. Make sure you have the means to actually power the unit. As I said, mine is a small unit and requires a 30A/120v outlet. Depending on if you do your own electric or not, there could be a pretty decent cost involved in installing the outlet alone. I was lucky to have a 125A subpanel ~15' from where I was putting the outlet in. This only required 10/2 wire. Much further and I would have had to step up to #8's. Also, if you go to a 4kw unit or higher, it requires 120/240v 4wire. Not much more cost involved there, but it is an additional conductor which again, depending on how far you're going can add up. Long story short, you're not going to run one off of plain old 15A/120v circuit.
Operating cost for our light in PA runs right around ~$75 for our running season. $0.13/kwh, 5 hours a day over 30 operating days @ 3720w. It breaks down to about $2.50/day in electric alone. A 4kw unit will run ~$3.25/day. Every few years you'll need to replace the bulb. An imported 3kw bulb runs ~$300, a "name brand" 3kw short arc runs about double that. It gets a little bit more expensive the higher the wattage.
If you're considering one, I would recommend going simple with it. I'm not going to name any names but there was a 4kw unit being demo'ed at Transworld. I'm sure it's an ok unit but consider things like the reflector size and additional electronics. The reflector in that light was ~8", half the size of my unit. There's was also 1000w more powerful, but yet my unit puts out a beam that is CLEARLY more visible. There's had DMX control, colored filters, etc etc and that's all good and fine, but DMX adds complexity that isn't needed in an outdoor environment and colored filters SIGNIFICANTLY cut down on how much light gets put into the air. I'm good friends with another local haunt that used one similar to the unit at Transworld and he hates the thing. He paid a lot more for additional features that he thought would be great (most notably DMX and filters) only to never use them because it isn't needed. Eventually something in the control board broke down and the parts aren't made anymore. They now have a few hundred pound paperweight. Filters, DMX, etc are great for indoor arena shows, not so awesome for searchlights outside.
Ultimately I'd love to get my hands on an old 60" WWII GE carbon-arc searchlight, but those are incredibly expensive, both to buy as well as to operate. A cheap used light will run you ~$25k. Most facilities don't have the excess power for them, so factor in another $20k for a suitable generator. Then factor in another $30/night for carbon rods and another ~$30/night for diesel fuel. Yeah.. But oh man, the beam off of that thing!
Hope this helps!
Operations Manager & Technical Director