Can anyone clear up as to why you have to replace a perfectly good piece of metal, the intake manifold, that's dirty (clogged up with soot)?
If I were in this situation I'd ask the mechanic to show me the part have him explain the reason as to why he feels the need to replace it (aside from convenience at my expense) then, if unsatisfied with his answer, take it off of him and find someone that would clean it (after giving it a go myself) for a fraction of the price of a replacement part. I don't see why a soak in a dissolvent bath and a bit of elbow grease couldn't get the job done (hell, even coca cola has been known to unseize engines). If it you can't get it to a respectable level yourself then you can always go with plan A, get a new part.
I find it very beneficial when replacing any part on the car to have the replaced part returned to me. I always keep the replaced part and have, many a time, gone back to that part and pulled of a piece to replace one that has worn out, saving myself money. It also sticks in my craw when I know that the mechanics take the safe and easy route by replacing a "faulty part" then keeping a good part that only needed a little TLC. Then there's the manufacturer who makes you replace a whole part when replacing a small insignificant part, such as was in my case, the dust boot on one of the front axels covering the outside U joint. I knew there was a cheaper way to go about it but just couldn't get it together in time, and, as I found out later, I managed to locate a parts dealer who sold universal dust boots at a small fraction of the price. I kept the original axel, then, lo and behold, the U joint wore out on the opposite side. We pulled the U joint off the old axel (much much easier than I imagined) cleaned and regreased it and replaced the faulty one with no problems since, cheap and cheerful.
I suggest you keep the replaced intake manifold and give it a go at cleaning it at your own time and pace, by doing things in this way I've learned a lot lessons, one of them is never trust a garage 100% with my money.