Ryan,
It depends if you're planning on keeping LPS/SPS coral, or not, or any calcifying inverts (such as clams, etc).
You will always want Ammonia, Nitrite, Nitrate. You'll check for these probably daily while your tank is cycling, to keep an eye on it. Once your first cycle is complete, and your skimmer is pulling some good skimmate out, I've gone to checking these weekly, after my 3% water change.
All Corals (to my knowledge) are sensitive to Alkalinity, so you will want this test if you plan on keeping them. As well as Phosphate.
Alkalinity also helps to control the pH in your tank.
If you plan on keeping LPS/SPS, want Coraline Algae or awesome Maxima/Crocea clams, you'll want a Calcium & Magnesium test kit as well.
The way I understand it, Alkalinity & Calcium are interlinked and both are needed for calcifying animals/life to grow as Alk helps the animals to absorb & process the calcium into their skeletons/shells. Magnesium is needed to bond these two effectively.
Yes, it's a lot of test kits. And yes, you'll be testing a lot in your first month, or two. But after you get everything under control, you can start backing off the testing so much and go with mostly weekly testing, or testing when something seems off in the tank. But it really all comes down to what you want, in your tank. I'm not thrilled with the money either, but I want a really awesome, and very successful tank. So I'm willing to put in the effort, and the money to do it. And, hopefully I'm doing it right.
There's also test kits I've seen out there for Iodine & Strontium, among others. But I haven't purchased/used those yet.
