How many tests, on average, do you get from the kit?

There are 456 drops in an ounce. This kit has 3/4 of an ounce of fluid. 3/4 times 456 = 342. If you average 15 drops (3000 ppm) per test than you can take 22 tests. If you average 20 drops per test than you can take 17 tests.

how do you use the drops ?

This is a drop based test kit. The instructions are very accurate and clear. You will have to swirl with one hand and add drops with another hand carefully. It is a wise idea to purchase the speed stir with magnetic bar (about $35). Mine has been around for 3 seasons and the batteries are still good. Instructions are here as follows: http://www.tftestkits.net/Test-Kit-Instructions-10.html Bottom TF-100 Instructions (same way) just different kit

Can this test kit be used to test salt levels in a koi pond?

Yes, Bev, you simply take a small sample (approx. 10 ml) from the koi pond and then perform the test...it takes less than a minute and is easy to tell when you reach the endpoint. If I had a koi pond, this is the test I would use for accurate and easy salt level readings. And what citrus guy said is true also, in case you need to keep your salt levels below 200 ppm. You might also want to invest in an inexpensive handheld TDS (Total Dissolved Solids) meter. Adding any chemicals (salts, pH adjusters, etc.) to the water will increase its TDS, and I'm pretty sure that TDS is also related to osmotic pressure (gill stress & health).

I just purchased one of these on amazon last week for $30. i'm not a math wizard, but what makes four of them worth $468.00???

Be careful with purchases like this, as you are better off on a pool website. The only issue with the salt test, is that you will lose 90% of the reagents. I no longer test with the kit and use the test strips, which work pretty well. Taylor makes great products, but this one is just not worth it. You will only need to test for salt 2X per season or every 3 months if in a hot climate. I test once per year in NJ. However, the Taylor full kit is well worth the money, or even the TF100 Test Kit from trouble free pool.