I made the rogue in November 2004 and sold it mid 2008, so...
42 months (3.5 yrs) @ $15
$50 for the game
$40 for TBC expansion (I think)
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$720 invested
The Paladin had a low amount of time played since he was an alt, but was in mostly season 2 gear (healing and dps) with T5 equivalent PvE sets for healing and dps, T5 equivalent tanking set. This was around the time of late Season 3/Black Temple. Oh he also had a season 2 gladiator mount. This character was on the same account and was the one that sold for $400.
The rogue was in 8/8 Tier 6 with the majority of his Sunwell upgrades and a mainhand warglaive. I think he was missing 1 piece of brutal gear (season 4). He also had a gladiator mount. He had over 200 days /played since he was my main for close to 4 years.
I used the money to buy a 22" LCD, a Radeon 4850, 4 gigs of RAM, and a couple games (Warhammer Online, Assassin's Creed, Call of Duty 4) and probably some other crap. Oh plus a new copy of WoW haha. By the time I sold the rogue I had a second level 70 rogue in greens on the same account. When I sold the first rogue, I transferred the fresh one onto a new account so I could level him to 80 in Wrath of the Lich King. Though I rerolled alliance and now have an 80 death knight and a 67 rogue.
EDIT: If I really wanted to, I could probably level a character 1-80 in less than a month, then another month to gear it out in heroic gear, then sell it. I'd probably make $500 tops, which isn't much for 2 months of work. The sales were a nice chunk of change though for work I would lose with the new expansion anyway, since leveling to 80 makes your old gear obsolete.