Here's one from Matt Buser over at Yahoo. It's old (06-07) but better than nothing.
It's not iron clad or anything, but here are the average scores for third place from the competitive roto leagues I played in last year (industry guys yadda yadda):
FG% - .469
FT% - .471
3's - 606
Reb - 3997
Ast - 2263
Stl - 701
Blk - 472
Pts - 9981
TOs - 371
That's for 12 man leagues with 10 active roster slots. If your league is set up differently just divide the above by 10 and then multiply by the number of active roster slots you have. If you\re playing in a less competitive league the numbers for third will be substantially higher. IN leagues like that you have a handful of guys who are substantially better prepared on draft day and through the season which tends to concentrate stats in the top third of the league. If you're one of those guys you're all good. The numbers in the Buser article skew way higher than most leagues I play in, but they're based on public league scores, which just goes to my point about stats collecting at the top in those leagues.
I went with third place because if you manage third in every cat you usually win the league.