I found a mercury racing hp500 for sale cheep and I'm thinking about upgrading

It is a 2000 502mpi with 345hrs

I am going to use some of the accessories on the 502 also like the power steering pump and some of the brackets

What is it worth

Any ball park idea?

Why don't you tell us what "cheap" is?

just remember the well known equation

intial cost x making it right cost
--------------------------------------- = actual cost
square root of perceived cost

What I have "seen" them for sale in the passed was 3-5k on a good used engine. List it for sale at a price your comfortable at and see what happens. Take offers, worst thing you can say is no. Also boat stuff seems to hit there peak prices around the 4th of july, so it might be on the downward trend now.