I found a blog that claims that the spell haste budget is .934 (Passive Haste studies.). They are comparing a healing bracer versus a dps bracer, and the math looks ok...
I thought it might be cleaner to compare two healing items. I took the [Swiftheal Mantle] and the [Amice of Brilliant Light[/item]. I liked these two items because they both use the same slot, they are of the same iLevel and same material, have the same armor and +healing. The only differences are:
The Amice has
-27 Spell Haste
We can then calculate spell haste if we know the correct weights of those other three stats.
Calculating for spell haste h where i is int weight, t is spirit weight and l is +healing weight. According to wowwiki again, spirit and int are 1 and +healing is .455, we get that h = 1.06.
While these numbers are a little closer, both are pretty far off from the numbers I found on shadowpriest and elsewhere. Is there a more official crunch than this? Is it possible that spell haste has a different budget on healing items than dps items?
Spell Haste Rating is a "combat rating" and therefore is worth the same as all other ratings and base stats (excepting Stamina), i.e. "1". Items don't always utilize their budget to 100% potential, they just get as close as possible, mostly without going over.
What you failed to read on the wowwiki page was that the itemization budget is not simple addition. You must take each stat times its weight to a power of 1.7095, add that up, and take the sum to the 1/1.7095 power. From here, there are various modifications to arrive at item level.
That said, stated item level and calculated item level don't always agree. Some items are underbudget.
Conventional wisdom (again, from the page you looked up int and spirit weights) is that all ratings have a weight of 1.