An S2 in visually excellent shape that runs well enough to ride can go for around 10k ball park. If you don't have the to touch the engine and no panels are missing it's probably worth it if cheap. The $$$ items for the S2 are actually not that bad nowadays, as there are plenty of sources for them.
Usual suspect for a resto bike:
> Seat: http://britishbikebits.com/dual-seat-wi ... BcHqfldVTI
> Tank: http://www.ebay.com/itm/NEW-ROYAL-ENFIE ... e5&vxp=mtr
> Mufflers/headers: http://www.hitchcocksmotorcycles.com/pa ... _Footrests
> Fork Tubes: http://britishbikebits.com/main-fork-tu ... BcJ4PldVTI
> Gaiters: http://britishbikebits.com/fork-gaiters ... BcKHfldVTI
> Fork Rebuild: http://britishbikebits.com/fork-bush-se ... BcKIPldVTI
> Harness: http://britishbikebits.com/cloth-wiring ... BcK3_ldVTI
All of that is ~$1800 shipped. Tack on ~100-150 for cables, $100 for a battery, $200 for tires, $80 for brake shoes. To get it running simply and not affect value, tack on $300 for a podtronix and electronic ignition with coils. That sits you somewhere in the ballpark of the $2500 range.
Missing items cost a lot, so a quick inventory of missing levers, mounts (check the battery and air filter area), etc... will get you a good estimate there. Fenders and stuff are available for ~$400 for a pair of something in that range, depending on shame, easier to replace than rechrome. If rims need to be replaced, they're pretty common, so it's not that bad.
Engine wise, if anything is up, be prepared to spend a lot of money very quick. When I put together pricing for a Mk1, a rebuild is upwards of $1500 depending on what needs to be replaced (cams, pistons, guides, etc...).