If this StuG is as good as it looks, is made of resin and photo-etched metal rather than polystone, and has the level of painting, weathering and detailing that is depicted, I do not think that $300 to $350 is an outrageous price.
The HB regular addition vehicles were about $200, the HB Gold Edition cost about $450-$650, the Figarti "shock and awe" Tiger with interior detailing was $300, the recent Figarti excellent Grant and Lee Tanks are around $240, and all of these tanks are made in large part of polystone, which is brittle and does not hold detail we well as resin.
K&C Resin and Metal tanks back in 1996-1998 cost around $380 (although they were very-very limited production, so scarsity value plays a role). Certain recent K&C Strictly Limited models are priced between $200 and $299.
It seems to me from my very limited exposure to First Legion (only photographs so far - I have never seen their products in person) that, with regard to his figures, Matt is attempting to bridge the gap in both price and quality between todays high quality toy soldiers and true "St. Petersburg" Museum Quality Military Miniatures. The photographs of his figures appear to have both the superior sculpting and the more time consuming painting-style employed by the Russian manufacturers (Figarti also employs the painting style, but has yet to attain consistent quality in sculpting). Conversely, his figures also have a price ($49 per figure) that bridges the gap between toy soldiers (around $32 per figure) and St. Petersburg ($120 per figure).
From the looks of this StuG, Matt has chosen to bridge the gap between toy armored vehicles and master-modeler's armored vehicles. Accordingly, you have to expect to pay a price that also bridges that gap. At least that's the way I see it.