We didn't pay a fee for PDI on our first two RVs as far as I can remember. We offered the dealer a price and they accepted it. If they charged a PDI, it didn't effect the bottom line.
On our current motorhome, they did charge us a fee of $1,500. However, that was included in the bottom line price. We asked them what was a realistic price of the motorhome. They told us, the last one they sold went for this much and that is what they'd like to get for the one they wanted to sell us. We were happy with their offer. They added PDI afterwards, but lowered the price of the MH so the bottom line would not be affected. The only thing we paid extra for was sales tax, licensing and documentation fees.
-Michael