before you go spending a lot of money, i would try converting your audio file to WAV (or getting an app that can record WAV files) before loading to iNat.
Voice Memo records audio in an MP4 container, and when you upload an MP4 file, iNat probably converts your MP4 file to one that has lossy audio, even if you recorded lossless audio. more about that here: https://forum.inaturalist.org/t/issues-with-quality-of-sound-recordings/52178/17.
if you can convert to WAV, iNat won’t do any extra conversion, and the quality of your uploaded audio might be better. it’s a little cumbersome of a process, but i figure it’ll be no worse that whatever process you planned to go through by buying a sound recorder.
if you still want a separate sound recorder, i would recommend a Zoom H1n. i see these going for as low as $60 new (not quite $50, but maybe there will be deals when Thanksgiving rolls around, or you can buy a used one now for <$50), but you’d probably need to spend a little extra to get a good wind screen for it, too.