Rathan/Vertexx - Unless Dharuhk decides to Houserule it, the way item stacking works is thus:
Take the most expensive function.
Add 1.5X cost of Each other function.
Double the cost (In addition to the 1.5X, for a total of 3X if its not allready the most expensive)) of any function that would be out of place in that slot. (IE adding +6 Str to boots)
EXAMPLES:
Ring of Feather falling/Evasion = 20,000 for Evasion + (2,200X1.5)=3,300 for Feather Fall = 23,300 GP
Boots of Ogres Speed = Belt of Ogres might +6(36,000X2*Wrong slot*)=72,000 + Boots of speed(12,000X1.5)=18,000 = 90,000 GP
Stacking weapons/armour is a lot trickier b/c of the way they do the math on them, for those I'd suggest getting the DM to figure out what he thinks is an appropriate price.