My estimate on LS swaps used to be $2000 plus the cost of performance or maintenance parts, plus the cost of transmission but including a junkyard 5.3. That would at least get into the ballpark. Now, thanks to inflation etc, I think you should probably start at $2k plus your engine and transmission costs, assuming you are going for stock power level (and there's ZERO wrong with that).
Baffled fuel tank and new pump/sender, something like nylon fuel line or making the factory hardline work, a reworked factory harness and modified tune for VATS and various delete (again, does not include performance tune), adapter plates, and misc pieces for hoses, intake tube, radiator, cast iron manifolds, etc. That's probably an aggressive budget for most, but other guys are going to say that they did the same swap for less than $1000 all-in, so I think it's fair. That's no labor, used/wrecking yard parts, and not much in the "might as well do this" category, but it will get you running and driving just like the 180k mile Yukon or whatever you pulled it out of...
When you look at swap budget breakdowns, you really need to look at it from a Required/Performance/Looks breakdown. If you want it to look nice, then you are going to add $500-1000 for breaking the engine down, cleaning everything, painting the block, reassembling with new gaskets, shaving the factory intake manifold, nicer filter setup, braided fuel lines, etc. If looks aren't a major factor, then most of the parts attached to it in the junkyard will do just fine, just spend a couple minutes with a can of Gunk to keep your hands cleaner while you work on it. That money isn't part of the swap, it's part of dressing the engine up.
Same for performance, if 300 hp is fine, grab any 5.3 and have fun. Some folks will have $5k in cam/heads/intake/headers etc. and call that part of the swap cost, where it really isn't. Making a plan is critical, most $10k+ swaps will have had a point in them where the owner changed their mind and subsequently had to re-buy parts because something didn't work with another part of the build, or a case of the "OOOOH! Shiny!" hits and they ditch the 4L60e they just built up for a T56 or something like that. Of course those can happen, and can be a good change. But don't let the total budget for something like that scare you, that's a Change Order in the business, and should not affect creating an initial budget that you plan to stick to.
If you stray farther and go into "non-standard" swap engines, you will be in uncharted territory. It's entirely possible that the factory manifolds from the Camaro V6 will fit in a truck chassis, and simultaneously possible that no combination of anything previously made will clear in your truck, and a $2000 set of custom mid length headers is the only option. Same for transmission options, maybe you can stab a regular driveshaft in and be good to go, or maybe you have to find a driveshaft from a 2008 Solstice GXP and mate it with a 2004 ElDorado halfshaft to make things work, and it's hard to say before you just dig in. Obviously I'm just making that up, but the point is that you don't know what you don't know on a one-off swap.
If you like the thought of a 6 (and who doesn't, especially if the truck was originally a 6 anyway) check out the Vortec 4200 straight 6 from the Trailblazer. It has a little more aftermarket support for swaps, and can make similar power to stock 4.8/5.3. There's a few folks running them in square's, but it would still be on the front edge of things. There is more support for the swap in general than for any modern GM V6's, but still much less than for a V8. Being a truck that was designed for a straight 6, I wouldn't think you would have too many fitment issues, but part of the fun is finding out...
