You are pushing your luck a bit if you wish to run 500 on any HR without a build.
IMHO you are relatively safe up to 460 on a HR but on thin ice at 500 and above.
However, to answer your question, unless you can acquire an HR engine and gearbox and all the other stuff that you will require at a reasonable price then the DE build will be cheaper.
Remember that you get an HR engine in the 350 as well as the 370, however I think that I have answered your question mate.
Alex.