You have mentioned that your roof is a terror and the previous installer sweared the whole day while they were in the roof.
Now what is a fair price for installing six panels on a tiled roof pulling the wires and getting out of there in under 3 hours vs mounting the same 6 panels on your roof and against the wall making sure no water can get through the holes and spending a full day or more. Should the cost be the same? What else is on the quote, what rails/mounting structure do they use. Deduct all the materials and what do they charge for the labour alone and how long will it really take? I can’t say if they are expensive without seeing the complete quote. And then again, there is the I know someone cheaper part, that won’t ensure that there is no leaks, use the correct rods through the wall that will not rust away over time landing your panels on the ground one morning when you wake up and 1000 other shortcuts. Are they still expensive? I can truly not comment as I haven’t seen the site.