Yes, an LS swap will cost a couple of thousand dollars, but so will the aftermarket EFI system once you're done with a fuel system and all the odds and ends. The difference is you still have old iron SBC heads, iron intake, tiny cam, block exhaust manifolds, etc. In some cases, you still have a distributor, V-belts, and other old stuff. If it's built right, the SBC has a slight advantage in low-end torque, but the LS will destroy it from the mid-range on. A 6.0 with just a cam swap has been shown to make 2x the HP the TBI engine was rated for.
Ultimately, it's your truck and your decision to make. A TBI V8 is the period correct powerplant and people have been getting all the performance and reliability they need from them for decades. There are many headache stories with all of the aftermarket systems and also glowing reviews. You don't really need one or benefit much until you're doing more extensive mods. If your expectations are realistic that you will pick up just a little power and/or just a little MPG, there's no reason not to proceed.