I found a way to grind bullets a little quicker. Rest, go down into the pit (spider floor) and collect the 15 bullets here. Now return and rest, and repeat. It takes less than a minute per run. Once you have a lot of bullets, trade them to merchent to refill your blood, then refill your bullets by grinding the spider floor until you're ready to continue the game.
Just a follow up, I lost my save game completely after dying on the spider floor by ramming into a spider. The game states 'Save Game Not Found. Start New Game?'. It's a real bummer coz I was enjoying it, but I might come back to it later with the grind in mind and try to quickly make up the progress I lost.