2000 Mazda 626 IAC / Code P1506

2000 Mazda 626 4-cylinder check engine light won't go off, Code p1506. I can't pass inspection till the check engine light is sorted. Local shop already tried the following - replaced IAC valve, IAC gasket, PCV valve, vacuum leak, adjusted/moved throttle body. (In addition they did a new muffle, new front left ball joint w/ control arm and sway bar link, oil change/filter). Any suggestions short of replacing throttle body ($900usd part)? Shop said something about plugging up one vacuum leak can cause a vacuum leak somewhere else. Is this true? Already sunk $1385.95USD into it, really don't want to sink any more $ into it.