When you start comparing 20B to LS1, the LS1 is going to be a much lighter motor, being all aluminum. The LS1 swap can be quite cheap if you consider how much you're selling, especially if your rotary is still in good, or at least rebuildable shape, and you've got a handful of aftermarket parts on it.

If you go the lazy route, you're going to spend about 5K on the swap kit. This includes all the mounting equipment/driveshaft, of course, as well as the custom-fit cooling system, intake pipe, the wiring harness modifications done for you (w/ tuned ECU if you send yours in), the fuel system (minus lines and fittings), and the custom headers that will fit the mounting kit. These are based on Hinson's prices, which are announced to be dropping shortly.

As for the big hardware, the cheapest I've heard of obtaining an LS1 is $2100. That's including it already mounted to the tranny, with the ECU/wiring attached, too. Average prices seem to be around $3500-$4000, depending on where you find them, but again, this includes the tranny/ECU/accessories. Its' definitely a lot of work, but I'm looking forward to it myself. Mainly for the opportunity to be able to clean up my engine bay and thoroughly go through and clean/organize my engine bay.
I'd say it's a good idea whether your engine is running, going out, or totally shot.