Occasionally you can get a 3-rotor for less than $4k, but it's rare.
First off... for the 3-rotor we'll assume $4k, no shipping added onto that or anything. You can't run the sequential turbos unless you have a whole front clip (w/ wiring, ECU and etc), so a turbo setup and full exhaust will be about $6k. The stock ecu, even if you do have it, won't be able to handle the upgraded turbo, so you'll need another $2k for an ECU. The hardware and fabrication to get the 20B to fit into the RX7 would only be about $2k, because you'll need a custom driveshaft as well. An intercooler, piping, BOV and etc will be about $2k. The entire fuel system will need to be upgraded, so that's about another $1k. The stock clutch and drivetrain won't take the punishment very long... $1k (at least).
That's up to $18k already, I overpriced some things and under priced other for the sake of having even numbers, but that should be about right. That's to have a 20B in a second gen RX7 that will be reliable and not ghetto'd together.
VAslr - show me a piece of cake install? Ask ANYONE on the RX7 boards if it's a "piece of cake". Sell your RX7 to a rotor head and buy a honda or learn what you're talkin about.
I MIGHT, might, be a little harsh, maybe you're just rotary ignorant. Do a little more research about the 20B before you consider installing one. It's quite an undertaking, not like droppin in a B16 or B18 into a Honduh.