Bronze rolls 10-14 and silver rolls 30-39 are the three ranges I've heard people have solid returns in.
I am an ebil markeeter and will steal your moneiz ...correction stole your moneiz. I support keeping the poor down because it is impossible to make moneiz in this game.
My best finds so far (see my thread http://boards.cityofheroes.com/showthread.php?t=245588 for some details) is for bronze lvl 35-39 and for silver lvl 40-44. I am sure all levels have great gems in them, but this is what works for me. lvl 40 because it gets rid of all the rare lvl 35 recipes that I wouldn't want. There still is shield breakers and kinetic combats you will miss out on, so you can try 35-39 silver, but I wouldnt try lvl 30. Also, the uniques that can be found at lvl 30 bronze just aren't worth it anymore...your milage on that may vary though.
Some things to consider:
See if crafting and buying the IO will even be worth that much of a profit margin for you.
Try and get rare salvage on the cheap, this will be the most expensive part of it all.
Be wary of proc's and see if the current selling price is right for you.
Remember, recipes themselves sometimes sell for a pretty good value...there are some pretty good uncommon recipes that sell for good amounts! (one is a resist set, then there is crushing impact and thunderstrike)
Use your drop tables found on the wiki to see what falls where. Do your homework!
Lastly, if your level 30 and above but not quite lvl 50, there is money to be made in lower level IO's for quality sets. If your lvl 35 and the IO would have been a lvl 50, it will be instead lvl 35. Don't charge less just because your would be lvl 50 IO is lvl 46...the difference is negligible.
http://paragonwiki.com/wiki/Silver_Roll is just one example....note the overlaping recipes at certian levels. note the cut off's for some recipes and start points for others. I myself, i just roll blindly :-)
I'm not sure if I'm posting this in the right spot but I was wondering what you all think pays off better? I usually go with silver but have not tryed gold rolls to many times.