Double it to be 100% complete in every way and sitting in it paddling on a nice warm sunny day. Around 2 grand.
The word 'kit' does not mean complete. I built a Shearwater. You won't get enough epoxy, enough wire, enough rope to even tie a pair of fisherman knots for grab loops and the 3/4" weatherstripping won't work on the recessed hatches- you will need to buy 3/8" and double stack it (this worked VERY well). Having built the boat and even being very happy with it, if I had to build another one, I still could not finish the boat with only the epoxy that comes in the kit, it is just not enough. The other stuff is pennies, epoxy is pricey and adds to the kit price.
You will use a couple/few hundred likely, in consumables...sandpaper, varnish, plastic sheathing, brushes, etc, etc.
If you need to buy power tools (sander/vacuum/plane)and stuff (mask/filters), that is another story. If you do need to buy a snader, I think everyone will agree the best type to buy is a random orbitol sander (ROS) with the round bag attachment that can be plugged right into your Shop Vac. This keeps the dust down to a minimum by a wide margin. You do not need to spend hundreds of dollars on one, i bought one from Sears fro aeround 60 dollars and it got through the job no problem.
When the boat is done, you will need paddling gear! Another few hundred at least. Don't forget you have to attach it to your car some way if you do not have a waterfront home and a cart of some sort if you have a long walk to the water from the car.
Being realistic, to see the day you are out paddling, you will have spent somewhere around $2,000, give or take. The good news is you do not need all the money up front at once, you can do it over time as many people do.
In the end is it worth the time, effort and money? The great news is YES! And many of us, if not most, find we even want to buy and build another one. And we have some left over supplies from the first build!
Here is a link to my Shearwater Building Blog if you want to take a look. These blogs are designed to be in reverse chronological order, so you will have to page back a ways if you want to start from the beginning.....
http://shearwaterkayak.blogspot.com/