Welcome to the world of RVing and welcome to IRV2. You will find a lot of useful info here as well as adventures across the continent.
I read the article below that said Rvs depreciate at different rates depending on what kind of Rv it was, travel trailer, MH, class a, b, c, etc. It also says that, "Starting with MSRP the depreciation is 30% driving it off the lot, another 10% at the end of the first year, and 6% for each year following."
So I would take the average MSRP and reduce it by 10% and see what they say.
Good luck on your purchase and that 45' Allegro will be a great home.