Used cabs are so hard to predict. I don't know, but bear with me on this line of thinking. So is that $500 total? But you already have a GK 212-II, that's a $599 cab, not a cheap cabinet by all means. It's actually a really good midrange cabinet already. I got some friends who would kill to have that cabinet because they're using a Rumble 150 combo to gig with. I'm not saying this to make you feel worse for not appreciating what you have. I'm just stating the facts here.
Say you somehow get convinced by my plugging of the TC K212 and end up buying one. I mean, is it really a tonal upgrade? Probably not. It's a sideways move at best, maybe some might consider it an upgrade, some a downgrade. And so will anything be like that, at the $500 used range. You might get more favorable dimensions, but will you be satisfied with the tone? It's not clear. And then what if you end up shopping for a third cab to satisfy your needs? It just seems like a bit of a waste to me.
So, once again, bear with me here. For instance, someone ahead of me suggested a Bergantino. Right now on my local classifieds, there's a Bergantino CN212 for $950. Which, you know, it sounds like a lot, but it really might be the very last cab you might ever buy. That truly is a fine cabinet. We all know that thing's not gonna sell for $950. It's going to sit on the classifieds for a few weeks at that price. Say you can talk that cab down a little bit. Now, you're talking about a cabinet that is both an upgrade dimensionally and tonally. This is completely hypothetical, because obviously your own local used market is different. But say you can find something boutique that is a tall 212 that can be had for $900 range, right? This might let you sell your GK212-II and end up being somewhere in that $500-range net. But who knows, right? The used market is really difficult to predict. But if you're savvy, you might come out a winner having upgraded your sound and your late night load-ins.
Like you, I am a working stiff. I'm a salaried worker working a desk job, I have limited income, and I don't exactly make money from music (I make a few hundred in a good month). But I like nice things. I only sprung the cash for a Bergantino (well, two) when I realized that the more I delayed the inevitable, the more money I'd spend on lesser products that don't quite do it for me. I haven't seen a single suggestion on this thread for an affordable $500 cab that's going to be a clear upgrade for you unless you aren't all that particular and you just want a taller, narrower cab, then by all means get the TC K cabs, they're great. Since you already have a solid gigging cab, if you're going to get another one, it might be time to see if there's something at another price range altogether, and not just assume that those cabs are for "those guys."