one local gun shop likes to try and make $50-$100 on most of his guns. $600+ glocks and all.
That's the key: It all depends on the shop in question.
If you look around the area I live in, the gun stores with indoor ranges have the best prices on firearms and accessories (ammo is its own subject). They don't need as much markup on the firearms, because range fees and ammunition sales (for the range) take care of overhead costs.
In this area, that means they're asking about 10-15% over their cost.
Stores that don't have a range have higher prices on firearms and accessories, but might have some other kind of "Ace in the Hole" to bring customers in, such as a massive selection of CC weapons, a (locally) well-respected gunsmith, a good selection of reloading supplies, or even just a salesman that's really good at teaching people how to use all the fancy new reticles in rifle scopes.
Around here, that means you're paying 15-25% over cost.
There are always exceptions, of course.
One of the most well known mall ninja shops in this area actually has a fantastic indoor range. It's .50 BMG rated, clean, well maintained, has great ventilation, has reasonable range fees, has decent ROs, and you don't have to wait 3 hours to get a lane. But... their firearms selection sucks, prices are sky high, their primary clientele are idiots and high school kids, and they're asking twice what other shops want for ammunition. Be prepared to pay 30-60% over their cost (sometimes 10-15% over MSRP), if you want to buy firearms or ammunition from them.
On the flip side, we have a local shop that has a terrible selection, a crappy gunsmith, no range, a disgusting shop (seemingly never-cleaned, former service station), wants MSRP for everything he sells (including used guns), and charges $75 for a transfer. He only stays in business, because he's been there for 20 years and built a loyal (albeit small) customer base.