oil on spark plugs possibly caused by tube seals/valve cover gaskets

I just had 97k spark plugs replaced on my 2010 Tundra 5.7, by my "backyard but certified" mechanic. He showed me the #2 and #3 plugs being "coated" with oil from inside the plug sleeve and believed cause was the tube seals not sealing properly. There was no indication of oil on outside of plug tubes or on cylinder heads. He advised replacing all eight at same time while valve covers are removed, as access is most of the labor. He "estimated" this might cost me $1700-$1800 at a dealership and he would do it for $900. Does this sound in-line to you that have had similar repair?
Toyota Service here, cannot give me an accurate job quote without doing $150 diagnostics charge based on someone else's opinion of problem and correction.
Any help appreciated!