I'm in a similar situation, except that the boat I recently purchased came with all cushions, dated and odor filled as they might be..
My boat is a 1976 C30, Hull #222. I originally planned on reusing the existing foam and recovering with Sunbrella, but after pulling all the old covers off today and seeing the condition of the foam, I'm thinking I may need to purchase replacement foam as well. In order to do so, I'll be taking measurements of the foam that I have and can share those when I do. My guess is that they will be similar to those that Dave provided.
$5,000 from Catalina sounds steep, so I have two additional questions, the second of which might help the OP:
1. Has anyone had success with cleaning slightly mildewed and/or dirty foam cushions? Any hope for mine, based on the attached photos?
2. What is the best source for 4" foam (preferably outdoor, open cell type) if I were to cut and cover new foam on my own?