New they are about $2300 + tax + shipping (Wolverine supplies). Used they depend pretty heavily on what's included. Basic rifle (rifle, 1 mag, HK tool, manual, maybe one accessory of some kind thrown in) can be expected to sell for $1600 - $1900 (Grey ones are cheaper then black ones).
Accessories cost alot, and some people try to sell a rifle with 10 mags, bipod, tri-optic rail, etc for $3000 - $4000. IMHO in such situation people would do better selling the rifle and accessories separately, since people that already own SL8 would want some accessories, and people that do not have SL8 get turned off by the high price.
Converted to G36 setup they cost alot more, with "chopstix kid" style conversions costing the most (and plug-in Hera Arms type conversions being cheaper, since alot less work is involved). I'd think that $3500 - $4000 is reasonable for an SL8 with receiver properly re-done into G36 style receiver (because good luck finding G36 receiver ends in Canada). With properly done barrel, bayonet attachment, flash hider, appropriate rails, and vented handguard (ie a complete G36 made out of SL8), expect them to be over $5000 (Barrel on it's own would sell for over $1000 in hours). Keep in mind that the asking price for a G36 is about $9000 (selling price can be expected to be somewhat lower), and with properly done conversion that's what you're getting. Appropriate shorty barrel will be a premium too (recall that you can't legally shorten a non-restricted barrel into restricted barrel yourself - to be legal you need to send it to be re-manufactured, or import a real G36K or G36C barrel)
Something like that.