Among the liquid test kits, API is the cheapest one that I know but with shrimp keeping you would also need the gh & kh test. Sera has separate tests available for ammonia, nitrite, nitrate, kh & gh but it will cost around the same or more than the API kit. The cheapest option will be Tetra 6-in-1 Test Strips (then buy a separate test for ammonia) though they are not that accurate but will give you a fair idea with what's going on in your tank water.
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