Interesting question.
Purely as an investment, there is more money to be made outside of the very high end rifles such as H&H Royals etc.
IMHO, as percentages goes, there is more to choose from in the $10K-$30K range which will more likely gain than a $70K rifle.
Think of it this way, for a $20K gun to gain 20% in value up to $24K is more reasonable and palatable to a buyer than a $70K rifle moving to $84K over the same given period of time.

That said, of the "big four" (H&H, Purdey, Rigby, and W.R.) I like the rising bite Rigbys best.
Of course, $70K sounds like a lot of money, but when talking original (condition, chambering, excellent finish, excellent bores, etc.) pre-war Holland Royals, you may find they aren't as plentiful as you thought. You should be able to get a good one though.

Watch the auctions too.