Rule of thumb is: item with both main secondaries = base, item with only 1 main secondary = -5ilvl, item with none of your 2 mains = -10ilvl.
More realistically, download Pawn and it will tell you how much in +% the item is relative to your current one.
To follow up on Ukerric here, stat weights depend on the spec. Some specs have a secondary stats that are better than a primary stat (this seems like a bad design to me). For example, Boomkin have haste over int, and Arms Warior has two secondaries over Str, I think.
There are various ways to get your stat weights, with various levels of accuracy. Stat weights will change depending on current stat levels, so, for example, Vers might be the best up to 10%, then if falls to last.
The most accurate way, that I have seen recommended, is to use SimCraft (
SimulationCraft - Download) and then load those numbers in game, into the Pawn add-on. Otherwise, you can use AskMrRobot.com free service to get a rough estimate for gear scores. The paid part of AskMrRobot may be more accurate, but I have not used it.
Here is a guide from Wowhead on Simcraft and Pawn, with pictures:
How To Use Simulationcraft and Pawn
Here is a condensed walkthrough to use SimCraft for stat weights:
- Go to the options main tab
- In options sub tabs, go to scaling
- check box "Enable Scaling"
- Go down to the bottom boxes and only select stats relevant to your class. (Say for Rogue, I check Agi, Crit, Haste, Mastery, & Vers)
- Go to the import main tab
- Select realm and type in character name
- Hit import
- Hit simulate at the bottom
- Wait for it to finish and automatically open up the results tab (this make take a min and use a lot of system resources)
- In the results tab, scroll down to the forth box that has the "Pawn string"
- Copy the string
- Back in game, open Pawn menu
- Click the spec of choice
- Click manual (let me manage scales) at the bottom
- Select Import
- Paste the "Pawn string" from Simcraft.
- Mouse over gear to see what the score is.