There is no website that shows average retail or dealer cost. Best you can do, find the floor plan that fits your needs. It might be available from several manufacturers. Then research dealer prices, on those particular makes / models, from places like RV Wholesale.com, RVtrader.com, etc.
You find, the same make / model, with same options installed, will be advertised with different MSRP's. That is because, the individual dealers set their own MSRP. They base it on dealer cost, dealer nstalled options, delivery charges, and dealer mark up. One dealer might say MSRP is 40K and he'll sell the unit at 35% off MSRP. That unit will cost 26K. You might see the same unit with MSRP 35K and the dealer will sell it at 27% off MSRP. That unit is also in 26K range.
The average is 25 - 30 percent off MSRP, but, some dealers push the envelope on determining what the MSRP is.