For under $100? Not a snowball's chance in hell.

There are any number of soil testers that cost less than $100.00 and give readings in percent, but whether they are any good is the question. Few are reliable enough to be of much use, and most all of them rely on an electric current to determine soil chemicals.

It looks like I am not really going to find an inexpensive way to get NPK readings by doing my own soil test, can anyone recommend a soil testing laboratory I could use?

Where are you? Each states university that has an Ag school may provide that service, although some don't. The link below provides a list of them throughout the United States

Here is a link that might be useful: USDA Cooperative Extension

You should be able to get a lot of lab analysis done before you cross the $100 mark. What kind of compost volume are you producing per year, and how many tests would you be running?

In 20 years of (suburban garden) composting, I've analyzed my soil twice and my compost once. Unless there are problems occurring, I figure if I put good ingredients into the compost, good compost comes out. So far it's worked pretty well. Of course I have no idea what your operation looks like, so I'm not saying you don't need to test, just sharing my experience.

That was just my point on the other thread, if you put good things in and you make compost yourself, you are ok, but if you buy it somewhere and have it trucked in then you should test it before adding it, as once it is added, it is really hard to undo anything to the soil. Unless of course, someone before you, added something bad, or there was meth lab waste located in your garden.

I think I saw that on H&G Network - "Make a Lovely Garden Meth Lab!"

Seriously though, I have used a multitude of imported compost products and have never noticed any problems in the garden that I could attribute to them. I look for good appearance and aroma, and I vary the types rather than using just one source. I've never felt the need to test any and all imported compost. YMMV.