Mivec RS is a bit special (Lightweighted version of normal Mivec).
There are 2 versions of Mivec RS
One with no aircond controller, no aircond compressor and no power steering - Rally spec
One with analog/mechanical Aircond controller - with aircond compressor and power steering - Normal road leagal RS spec. - Comes with Mivec-RS timing controller (normally taken out by half cut shop to sell seperately)
How much it cost = depends on Half cut shop. Lately Mivec Halfcut price has gone up to near GSR price range.
The possible problems that buying this kind of engine =
1. It has no history records of oil change (probably lots of stuff worn out)
2. It has been heavily abusive driven (racing used) in its previous life (more wear and tear which not known/detected in early stage of transplant it in to your car)
3. Some parts might have been change to aftermarket parts already by its previous sports owner (especially rally spec RS model) thus not easy to find replacement parts after it spoilt
4. The Gearbox might have some problem (worn out gear selector / syncroniser inside the gearbox might worn out). The next problem might be the Idler gear (the small gears that control reverse) might worn out due to abusive and wrongly perform reverse J turn. The last problem would be possible worn out LSD.
5. etc etc...
Hope that helps...thats is from my 2 cents knowledge when i do research on this type of engine last time. In the end i go for Mivec-MD engine power car due to cost and budget constrain. Anyway, here is my MIVEC-MD + the controller from the MIVEC RS pic (Its plug and tune and play).
http://farm3.static.flickr.com/2248/2105335999_eb90b6ab4b_o.jpg
P.S.: The best options is if you can find a good unit and trusted mechanic friends done checking it, it is a very rev happy and good NA engine to play with.