I just sold a 12 hole course this morning to a private school. Their cost for the baskets shipped will be $4,500. They school maintenance department will be installing the course and will be making the signs.
18 Baskets will cost between $5,500 on the low end to 9,000 on the high end with shipping. It really depends on which baskets you buy.
A good starting point for signs is about $1,000 from anyone of the Disc Golf companies that have signs as an option with their course packages. I believe Innova and DGA have signs you can add to a course package. Discraft doesn't. Not sure about Gateway.
Natural Tees just require something to make the front of the tee area. Rubber Fly pads, Concrete, Pavers or crushed rock are all possibilities. Crushed rock is normally the easiest for a medium or larger community because they are likely to have plenty of crush and run available at any given time...so additional cost is simply building the tee boxes.
Depending on the lay of the land, you might want to include the cost of a decent designer. I know most think they could design a course but you're better off having someone with experience. While I have sold a bunch of courses over the years, I've only been lead design on two courses I sold because I was able to get better designers with more experience the design job.
A good designer can be had for as little as $400 a day (plus expenses) with the average being three days work if the property is more open...it takes more time to do a design if more clearing is needed. A top designer can be tens of thousands of dollars...but then you get what you pay for.