I can't find it now, but I recall it being discussed on here not so long ago - Bluebell was cited as one which had benefitted, but it wasn't so good for others.
I dont think youll find there are any official collated figures. I did some research in connection with the business case for a possible extension to my heritage railway.
My evidence was basically emails from most of the railways in the south east that Id generally solicited from their representatives at HRA meetings.
The number of people arriving at Heritage railways by Public Transport is very low generally less than 5% by far the majority of passengers arrive by private car coach parties. Interestingly it appears that this is across all types of passengers even enthusiasts.
The number 1 railway for passengers arriving by National Rail was the Isle of Wight Railway at 8%. This is perhaps surprisingly low given the special circumstances.
The Bluebell at East Grinstead was still settling in folowing the honeymoon opening to East Grinstead but the figures are I understand OK rather than spectacular.
One General Manager that ran in to a national rail station opinioned that it actually ran at a loss.
Before someone mentions the Keighley I deliberately ignored that because its a Metropolitan Area that our railway couldnt replicate.
I also investigated through ticketing most railways opinioned that this actually cost them money and existed for political reasons rather than commercial.
The only railway that I found that said it was a significant earner was a liight railway in the northwest where the mainline toc didnt make any admin charge.