Recomendations for a 40mm or less diver at around $500

Glycine 39 Combat Sub: $525-560

I recall them having a gilt version (sim to pictured BB58) when they first launched but seems no longer. I think this blue version still pretty nice package. Good luck!

https://www.gnomonwatches.com/collections/glycine/products/combat-sub-39-metallic-blue-ref-gl0403

Vostok Amphibian is 40mm, but it's a weird watch, and you may have a problem with the whole supporting Russia thing, which I totally understand. Luminox Leatherback Sea Turtle is 39mm. It's also a weird watch that reminds me of the old Tag Heuer Formula One from the 80s. If you look it up you'll see what I mean. The hard part is the size. If you want to go up in size there are lots of options, like the Orient Komasu or several other Seikos and Tissots in that price range. 

how about steinhart ocean one, there is a plenty variant of those. sure its an homage but built quality and movement are solid

Raven and NTH has a few you may like 

Perhaps the Baltic Aquascape series?

 Or the Vaer D5 Meridian (currently 20% off on their site)

Lorier has some gilt black diver that fits your budget. U can find some christopher ward used at that price range too. Good luck finding what you like.

Octon Baltic Blue: https://www.youtube.com/watch?v=q39cgkRLWvs

San Martin homages are an option. Their SN004G is a 38mm vintage diver homage that's well under $500. Has a Seiko movement!

The Baltic Aquascaphe for sure. It's such a lovely watch.

Definitely Lorier Neptune or Baltic Aquascaphe if you can stretch your budget. These are probably my two favs for having the BB58 vibe without being a direct copy. 
 

Timex has an interesting take on the look with their Harborside Coast too. 

if all else fails, invicta 1953 should hit the spot. 

I like the Lorier watches, but their gilt black diver is $689. 

How about a titanium diver by RZE for $499? 
https://www.rzewatches.com/pages/endeavour-page

Orient Kamasu 2 in this particular color has vintage-type lume and indices. it is 41.8mm but 46.8mm lug to lug, so wears smaller.

For me, it's the Invicta 1953. 200mm, Nh35 and can be had for a little under $130 with a nice bracelet. 

I see there is a price increase for the new model. In my mind they were 400 usd lol https://www.themodestman.com/lorier-neptune-review/ a gentle used one should be well under 500 then

this is phenomenal at the price and they are constantly improving it. I honestly think it looks better than the actual Tudor because the indices and hands are bigger which is why I got the black one for myself ;)

I'm with Lorier if you like Vintage aesthetics and not against acrylic crystal, 

MAEN Hudson if you are into Art Dec

You can go homage route and choose Steinhart or San Martin SN-008 with Selita movement 

Or Nodus Sector Dive Pearl if you want a diver with explorer vibes 

Isn't it bigger than 40mm?

It’s Baltic Aquascaphe blue gilt for me.

I do like that Glycine Combat Sub too.

Seiko  SNE 575P1, 38,5mm.

Probably my favorite from my collection. 

The Baltic is the way to go .

Two modern ‘classics’

This Hamilton Khaki Navy Scuba is available for just over $500 and is 37mm. You didn't rule out quartz so I thought it was worth a mention.

I would suggest a Venturo - Skindiver. The case has 40mm but with the bezel it‘s 41mm.

AQUASCAPHE Steel Bezel SB01 has a nice vintage look in a 39mm case. It sits around 520 dollars but for 80 dollars more you can get a beautiful beeds off rice bracelet

Came to mention several brands already listed above. You could also go with a diver from Long Island Watch. A bit more modern, but solid choices and very affordable. I have their 40mm Bayport and am very happy with it. 
 

Please please please do yourself a favor and check out the newly released Squale Montauk (Long Island watches site). Fantastic little divers (38mm case with a slightly larger 40mm bezel, so they look and wear great). And 595$ new on a rubber strap that’s super comfortable. My only real criticism is the lug holes for the spring bar are tight to the case, so a curved spring bar would be needed for a nato strap.  But on the rubber it comes with or a replacement bracelet (mine is on a Forstner rivet dive bracelet) the watch looks and wears awesome!  Good luck on the hunt. 
 

I have to agree with watchguy75, Orient has several options under $500

Seiko Prospex SNE573 

• Solar powered quartz movement, accurate to +/-15 seconds per month
• 38.5mm wide, 10.5mm thick, 46.5mm lug to lug, 20mm lug width
• Stainless steel case, sapphire crystal, 200m water resistance

I happen to be selling one lightly used with a bracelet from an SBDN069 instead of the typical silicone strap. 

Skx009j