I probably have spent less than 20 million in inf on Heraclea (Inv/DM), and she has one set of Superior Might of the Tanker, is working on the other ATO, two sets of Kinetic Combat, and various other IO sets. She is softcapped at resistance to S/L, has defense in the 40-ish level to most everything but Psi, and I am generally satisfied with her performance. It helps a lot that I run a lot of TFs and buy recipes with merits specifically to avoid the AH. I also stockpile yellow and orange salvage and share it among all my characters through bases, again to avoid the AH.
Incarnate stuff is the other path to buffing your level 50 character, and inf pays a minor role there. Mostly it's about grinding out the threads and shards, and every character has to run this grind for herself.