A very simple refractive IR spectrometer is possible for under $1000 if you can live with 2-10μm range. I don't have a full write up yet but the basic design is an MLX90614 thermopile scanning infrared light generated by nichrome wire, focused with a zinc selenide CO2 laser lens (the ones without an AR coating are better but harder to find), and separated by a silicon prism which is essentially just a chunk of a broken silicon boule. It's very difficult to find transparent materials in this wavelength range, especially when you're limited to cheap goods mass-produced for some other application. Signal noise and speed will be much worse than a modern FTIR but it's usable when calibrated.