Price range: I'm thinking used, up to maybe $600
problem is I tuck in all of my shirts.
Well I felt like you opened the door wide open and then slammed it shut tight. 1st, that price range, used, will allow you to buy practically anything you would like. You can easily get a very capable sidearm for that money. Since you are so familiar with the 357 magnum I'd recommend you look at a j frame Smith revolver in either 357 or 38 Spl+p. But if you have your heart set on a 40, 9mm, or 380, I'd look at a Kahr, S&W Shield, Ruger LC9 or the like.
Now for what I consider to be the hard part. The tucked in shirt makes it tough. Tough to conceal and (more important) tough to access should the need ever arise. And as you know, if the need ever arises it will come about so quickly you'll be behind the 8-ball from the start.
I'd recommend pocket carry with a decent pocket holster like a nemesis or a mika. Problem is, you can't do this in jeans. In a pair of jeans, the gun may
fit in your pocket, but you cannot draw it quickly. On the other hand, dockers are perfect for this arrangement. The pockets are roomy and ideal for pocket carry of the right pistol in the right holster.
Now, here's the part where I make a suggestion you didn't ask for. Forgive me. Consider, just consider
, adjusting your attire and wearing your shirts untucked from time to time. This can readily be done with polos and can even be done with button up shirts by today's styles. This modification will open up your possibilities a great deal. I know it's an adjustment, but it is one I made without much fuss. If you make this change, you'll get into the world of IWB holsters and there are truly some great ones out there.
Let us know what you decide.