2002 Honda Civic too much?

2002 Honda Civic

I acidentally put diesel into this car, intially the cost for flushing the system 2 times, cleaned sparks, injectors, codes and roadtested 2x. The last statement is "everything is ok at this time. That bill came to $646.56. Now I am being told there is going to be an additional charge for further work costing $1800.00. For ! Part, I asked for an itemized estimate, which I havent gotten yet. But, she did say that it was an air sensor. Would the cost of 1 part for this car be that expensive? I have searched the web and havent seen anything, nor do mechanics I have know told me that it should be that expensive. I was also told that the part needed is very difficult to find and they had to order direct. I need help with this. Thanks.