That sign is worth more than $300. Here are some IGA signs that sold at Miller's for around $800, $900 and $1400. That was an auction, so price may be high, but your sign has better graphics (in my opinion), and is probably more rare than IGA (and older). https://www.auctionzip.com/auction-catalog/catalog_4RD3PDL743?keywords=iga
Here's a western state groceries that sold for $700https://www.ebay.com/itm/302760586150
The fact that it's not gas & oil doesn't mean much. There are guys who collect gas & oil, guys that collect soda, guys and farmers that collect farm & feed, guys that collect grocery & retail, guys that collect dairy & ice cream, and guys that collect all of the above.
I know these are not the same exact signs to compare to, but I would buy that sign all day long at $300.