Welcome to my world. I was in the same boat about a month ago. I've done some research and talked to a few members here to try and figure things out. So far it looks like a $10K project depending on your level of involvement with the install.
L28ET fits the existing mounts and you can reuse the majority of the items in the engine bay.
RB swap will require engine & tranny mounts, driveshaft, intercooler, radiator, custom downpipe & exhaust, upgraded fuel delivery, and a few other items. ~$1.5 - $2.5K
Items you'll probably need either way...engine rebuild kit, gaskets, rings.
Engine& Tranny combo $4K - $5K
Items from above $1.5K $2.5K
Misc. $1K - $2K
Total (estimate) $6.5K - $9.5K
Don't forget about the need to upgrade other items like your differential, brakes, suspension, etc.
I'm sure others with much more experience can chime in on this.