45 copies of the Indo-European Grammar have been sold one month after publication

Not bad at all for 1) a fairly obscure project, and 2) a book that can be obtained online for free anyway. Selling 45 copies of a book in a month is something that could only be done by Esperanto/Ido/Interlingua, so in spite of not having a single fluent speaker Modern Indo-European is managing to attract quite a bit of interest.

This reminds me of a study that I believe the auxlang and reconstructed language-supporting community should one day try to fund: ascertaining primary reactions to a (re)constructed language based on 1) an explanation of its philosophy and 2) its appearance. Some very well-constructed languages like Novial have had less success than they deserve perhaps for having an odd name (Ido fits in there too) or a seeming lack of raison d'être. Interlingua is easy to explain and it looks great at first sight, Occidental is fairly good at that too but a bit more of an acquired taste (the first question is usually "okay, this looks like Interlingua so what makes it that much better?"), Esperanto has a great creation story and trendy kind of counter-cultural feel, and of course Modern Indo-European has a real archaic and authentic feel that resembles that of Latin but more misty and ephemeral.

The study might also want to create some sample languages and check the response to various names and philosophies. Try taking Novial and giving it the name Interlingua or the other way around, test reactions to the name Occidental vs. Interlingue, Ido vs. something like Interlinguo, Ido when Esperanto is mentioned vs. when it is simply mentioned on its own, taking Ido and calling it a modernized version of Vulgar Latin, etc. Experimenting with enough variables should give an idea of what regular people like and don't like...and if the study is comprehensive enough it might even drive a few existing auxlangs to change their name or promotional strategy in accordance with that.