I just finished doing this on a customers car (TB48 on Gas). It had a slightly rough idle and you could hear it lightly popping every now and then out of the exhaust. Adjusting the clearance fixed this.
The 4.8 doesnt have the conventional threaded flat head adjuster with locking nut, It uses a bucket type lifter system with different size shims which fit into the bucket to adjust clearance. From memory the minimum intake clearance was .32mm, And the minimum exhaust clearance was .33mm. You use correct size feeler guages to slip in between the lifter bucket and the cam to check the clearance, if it fits you are fine. If it doesnt you have to measure the clearance using the different size feeler guages to find the tightest fitting one, This is your clearance. Write it down along with what cylinder and which of the two valves it is (valve 1 or 2).
Once you have done them all you have to remove the shims which are too thick making sure to keep them in order of what cylinder and valve they came off. Some say you have to take the cams out however in this case it was just a matter of loosening the cam bridges so that the cam would lift off the buckets, then i used a screwdriver and a magnet to loosen the shims from the buckets and pick them out. Once this is done you measure the thickness of the shim, and do a calculation to see how much it needs to be ground down or if you are replacing them, what size you need.
Calculation is as follows......
.33mm - .15mm = .17mm
2.23mm - .17mm = 2.06mm
Explanation:
Say cylinder number 1, valve number one clearance is .15mm, the minimum clearance should be .33mm......So you know that is tighter than specs. So you now remove that shim and measure its thickness. The shim measures 2.23mm, so you now have to replace that shim or grind it down. So you take the specified valve clearance of .33mm, and you subtract the clearance you currently have which is .15mm, you are now left with .17mm which is the amount you have to grind off your shim. So you now know in order for the clearance to be at minimum tolerance, the shim originally 2.23mm has to be 2.06mm thick. If you have the correct equipment to do so then you grind away checking with a micrometer until you reach the desired thickness and reinstall. However most ppl wont have the correct specialised equipment for this so you will need to buy replacement shims. With correct shim height of 2.06 still remaining, its just a matter of ringing an appropriate shop and ordering them.
Probably doesn't help with your need for an accurate quote, but explains things and makes the subject a little more clearer.