The code does consider any underground conduit run to be a "wet location", even if the wire is in conduit. Oddly, the cable WILL get wet over time due to condensation. You MUST run flooded CAT5 or "icky pick" cable, and run the 18 gauge conductors out of a conductor type with a "W" in the lettering, such as THWN or THW or MTW. Doing anything less will result in failed cable in several years. I'm not sure what the load is on the end of that 24 volt run, but if it's more than 500milliamps, you're going to have VERY SUBSTANTIAL voltage drop. If you post the load amp or wattage, I can calculate the conductor size that you'll need up upsize to to eliminate voltage drop. If this if for tilt/pan type stuff, your motors will run much slower and run a bit hotter with the lowered voltage. I'd run #12 THWN conductors, at a minimum for that type of run for 600-800 feet, especially if your camera housing has the little heater to keep the glass from fogging.
Prices vary A LOT from localle to localle. In my area, trenching (with a ditch witch type tool) in grass is $1 a foot, plus rocks at time and material. Trenching in blacktop is $5 a foot, plus rocks at time and material. Boring or shooting under sidewalks is around 100 bucks for a 3 footer. Flooded cat5 is around 1.45 a foot, and #12 THWN is about .06 a foot. 3/4" pvc conduit is about .25 a foot. My totally ballpark guess, without looking at your job is about $3000 without any major problems with site access or rocks. Your price may be substantially more or a bit less. Replanting grass or fixing blacktop cuts is something I don't do, so I have no idea on that one. Any backhoe involvement drives the price up too.